I gotta admit, I've seen plenty of stories where these two are the same person, but none with them being sisters. This idea came to me when I thought 'hey, what if Maria was hanging out with the Avengers then her phone rings and it's playing Let's Go To The Mall Today, and the spies are just laughing and being unhelpful while Maria is supper embarrassed'. I was on antibiotics at the time so it was weird.
When Robin Charles Scherbatsky Sr. found out his wife was pregnant, he was happy. He would finally have a son! When they got the news that it was identical twins, the glee was doubled. Then when it turned out to be two daughters he was certainly not as excited. He made an agreement that he would get to name one of them (he was planning on naming one Robin Jr., just like he'd have named his son). Genevieve wanted to name the other after her grandmother, Maria. When the two were born, they realized they couldn't take care of both, so Genevieve sent Maria to her cousins in Chicago.
…
It was a family reunion, and Robin was bored. She was ten years old, and there was no one cool around. That was, until a girl around her age she had never met walked in. She had gray - or maybe blue? - eyes, like Robin, and brown hair down to her shoulders, shorter than Robin's. The girl was clearly related to her. Robin walked up to her and tapped her on the shoulder. "Oh!" The girl twirled around, surprised. "Hi! I'm Maria! Who're you?" The girl had an American accent, unlike Robin's Canadian one.
"Robin." She stood awkwardly for a moment. "So… How exactly are we related, eh? My mother's over there." She pointed to her mother, who was drinking and speaking to an American cousin of hers, Sophie Hill.
"She's talking to my mom. So are we… Cousins? Our parents are cousins, so what would be the term for that just be cousins as well?" Maria looked curiously at the girl who looked so much like her.
"I dunno. How old are you, anyway?" Robin was wondering because honestly the only other person close to her age. Well, there was her sister, but Katie was so much younger and a child and not like her.
"I was born on June 15th 1982. Ten years old. You?"
Robin stared for a moment, stunned. "I was born on the same day. We do look really similar, eh?"
Maria looked just as startled as Robin probably did. "Are- Are you saying we're, what, twins separated at birth? Because the signs are all pointing to that."
Robin smiled weakly. Of all the people to end up being siblings with, Maria didn't seem too bad."I-I think so."
After that, the two became close, sending letters to each other and phoning whenever convenient. Eventually letters turned into texts and emails, and their ability to communicate instantly only strengthened their bond. The two of them didn't really talk to their parents about the whole 'twins' issue, mostly because their parents were the ones who raised them, and neither particularly cared who was whose biologically. They were twins, and they had their own parents, who each of them cared about and loved. When Robin's parents separated, she ended up coming down to visit Chicago and realized she kinda liked the states. Robin promised Maria she'd move to the US one day, when she was a famous actress. Maria wasn't particularly interested in stuff like that, she'd always wanted to join the Air Force. Neither really cared about the career differences. Perhaps it was because they were twins, but the two never really drifted apart. They always stayed close.