It's a twin thing.

After three attempts they were beginning to lose hope. They had traveled to two worlds where there was an impending attack on the Goa'uld, one where Kate was just a baby and another where she had been sent through the gate to a beta site. They had traveled to another where Kate didn't even exist, Janet not being involved with Daniel there. They knew they were running out of time and were expecting the worst when they traveled to the world they were now in.
" Here we go again." Jack commented as they stepped out of the mirror.
"Well it doesn't sound like there's a Goa'uld attack." Sam commented looking around at their surroundings.
An alarm went off as one of them tripped it.
"Looks like were soon gonna find out." Janet told them.

A few minutes later they had been taken to the briefing room. There General Jack O'Neill, Colonel Samantha O'Neill, and alternate versions of Daniel and Janet joined them.
"So guys… okay this is weird. I assume you came through the device for help. What can we do for you?" General Jack O'Neill asked the group.
"I'm a General…cool." Jack commented. "Sorry." He continued looking at the table, suddenly finding his hands very interesting.
"I'd like to know that too." The alternate Janet told the team, a worried expression crossing her face when she saw the condition of her counterpart, and the fact that Daniel wasn't there with them.
"Do you have Kate here with you?" Sam asked the couple.
"Our daughter? Yeah of course. Why?"
"How old is she?" Janet asked.
"Nineteen. What's this about?" Daniel asked.
Sam looked over at Janet, a sad expression crossing her face when she saw the tears welling up behind her eyes.
"Our Daniel's in trouble. He went on an away mission and was struck down by a virus that has damaged his kidneys beyond repair. Dialysis won't work, as the damage is so severe. In order for him to live he needs a transplant, and he needs it as soon as possible."
"And you want Kate to donate one of her kidney's to him." Colonel Samantha O'Neill asked.
"If she's compatible yes." Janet replied.
"Are there no donors on the base?"
"No. Two people who are compatible are too sick themselves. The best bet in these cases is to have a family member donate a kidney. Daniel's Grandfather Nick is on another planet, our child summer is just a year old and we don't have Kate in our world."
"How do you know she'd be compatible?" General Jack O'Neill asked.
"A year ago to this day an alternate version of Kate came through our quantum mirror with Summer, who was then only a week old. Her parents had sent her to that universe when her home world was under attack by the Goa'uld and from that one she was sent to ours with Summer when their world was invaded. Kate was fatally injured and before she died warned me of this and that she was the only one who could help. Now Daniel's dying and she's our only hope." Janet told them.
"Look we're not trying to force her or you to let her do this for us. We would totally understand if you said no. But Daniel's dying. We weren't even authorized to come here but we couldn't just wait for a donor when he's as bad as he is."
The alternate Daniel looked at his wife.
"Honey I think we should let her decide. If that were me sitting over there I would be praying for my counterpart to let me ask Kate. I couldn't stand to lose you, it'd tear me apart."
"And if you were in Daniel's shoes…" General Jack O'Neill asked his colleague.
"I wouldn't want to die. Not when I have a family and especially not when I have Janet."
"Then we can ask?" Sam asked them.
"You can ask." Her counterpart replied.
Just as they were going to get up a small blond haired girl ran into the room, a little brown haired boy hot on her heels. Following them came more footsteps as a young woman in her late teens entered, her hair dark brown and falling around her face in waves and her eyes a deep chocolate brown.
"Sorry guys." She said to Sam and Jack as the young girl jumped into General Jack O'Neill's arms and the young boy jumped into his wife's.
"Guess they got away from me… okay this is, well, different." She commented looking around the room at the counterparts of the people she knew so well.
"We were just coming to find you sweetie." The alternate Janet told the young woman motioning for her to join them at the table, which she promptly did.
The children who were in their parents arms, just as perplexed about the situation as Kate got up and walked over to their parents counterparts looking at the amazed.
The young girl lifted up her arms to Sam, wanting to be taken up on her lap. At first Sam looked wearily at her counterpart but then looking at the child's sweet face she picked her up and sat her on her lap, the child wrapping her arms around Sam and closing her eyes. The young boy meanwhile was busy climbing on Jack who was laughing.
"Sorry about this. Kellie, Matthew…" Sam called.
"They're okay." Jack told the alternate Sam who sat there smiling affectionately at the antics of her children.
"So what's up?" She asked looking around her.
"The kids asleep Kate?" Daniel asked.
"Yeah. Summer's being picked up by Uncle George and he's dropping her off later so we can all go home together. Cass dropped off Aidan and Jay half an hour ago, they're asleep in my quarters and Teal'cs got Danni."
"Exactly how many kids do you guys have?" Jack commented to their counterparts.
"We have these two and a sixteen year old called Grace who's at school." General Jack O'Neill told him.
"While we have Cass, Kate here, Summer, Aidan, Jay and Danni which makes six."
"Whoa." Sam commented.
"You try looking after them all." Kate commented.
"We actually have something to ask you Kate and we need you to listen because it's quite serious." Janet told her.
"What's up?" the young woman asked.
"In our universe Daniel got sick on an away mission. He was struck down by an alien virus, which has damaged his kidneys. Do you know what dialysis is?"
"Uh yeah, I help mom in the infirmary. I'm training to be a nurse here at the SGC."
"Well the damage is too severe for dialysis to work. Daniel needs a kidney transplant." Janet told the young woman.
"And you need me to donate one of mine."
"If you're compatible yes." Jack told her as Matthew settled on his knee.
"Look we don't want to force you into this, that's the last thing we're trying to do. We totally understand if you say no." Sam Carter told her, bouncing Kellie up and down on her lap.
Kate looked at her parents.
"I don't know." She told Janet. "Do you realize what you're asking me?"
"Of course we do sweetheart." Janet replied.
For a minute the teenager sat there, quietly studying Janet's face.
"How far along are you?" she asked.
"What?" Janet replied.
"How far along are you?" she repeated.
"About three months I think… how did you?"
"Look my mom and my dad have six kids, five of them their own by birth. Now I don't know what my mother was like during her pregnancy with me and I can't really remember with Summer but I remember with the younger three."
She replied as Teal'c joined the group, handing Daniel and Janet's five year old to her parents after taking her for a piggyback around the base.
"Strange." He commented sitting down at the table with the rest of his colleagues.
"You got really bad morning sickness coming on right and you're tired a lot."
Janet nodded.
"Thought so. Aunt Sam was the same when she was having Kellie and Matthew. I know when one of you guys is pregnant just by looking at you and you most certainly are." She told Janet.
Kate took a deep breath. "I'm really dead? In that other reality?"
Jack nodded slowly.
"And I died giving Summer to you?" she asked Janet.
"Yes you did."
She sighed
" Then I'll do it. If I match I mean. It seems like the Goa'uld are destroying us in every reality and before they come here I want to do something good. Besides my parents have been married for just over nineteen years and they're happy…" she continued making them smile.
"I want you to have the same chance." She finished then looking at the table.
"This is a big thing Kate honey, are you sure?" Colonel Samantha O'Neill asked.
"Yeah I'm sure. When do you need me?"
"We need to test for compatibility as soon as possible."
"Then lets go huh." She told them getting up.
The rest of the group followed suit, making their way to the infirmary, Kellie in Sam's arms and Matthew in Jacks, Janet holding Danni who had been handed to her by her counterpart.
"Thank you." She whispered to Kate as they were making their way down there.
The young woman smiled.
"Thank you, for loving him enough to do this. You guys are really great together Janet, you just remember that." She replied watching her little sister play with Janet's hair.