I don't know what shoing was like in the 1930 and 40s so this isn't historiclly correct.I don't think they had pads or pregnancy tests but I've added them to make the story make sense.
I have sort of combined options 3 and 5 but not too much.

Sara flopped down on her bed they had left eygpt two weeks ago and after a quick stop in arabia they were finally back home. The scarab necklace was still round her neck. She hadn't taken it off and when Alex had asked her to she had pretended not to hear him. And when Rick had demanded she had refused. Evie had had to calm him down reminding him that at east they had her away from Imhotep. Imhote she missed him it hurt every time she thought of him and she couldn't stop her dreams were of eygpt and being his queen.
Suddenly Sara feltsickand ran to the bathroom just in time. Luckily she hd an ensuite. When she was sure she wasn't going to be sick anymore she went downstairs. Sara told them she hadn't done anything more than a kiss and had only gone with Imhotep to save her brother and as she had only been gone 2 days and come back freely noone doubted her.
"Sara guess what" Jessica squealed running in with Alex both with large smiles on their faces. Sara couldn't help the pang of jealousy in her heart despite her love for both Alex and Jessica.
"We're getting married." Alex said grinning. Evie lughed and hugged them both as did Rick.
"Can I be a bridesmaid" Sara asked hugging Jessica but she couldn't hel wishing it was her and Imhotep she tried to imagine Imhotep in a suit and smiled.
"Of course you can be maid of honour." Jessica told her laughing.

Sara ket being sick in the mornings as days past. But she managed to hide it, thinking it was just a slight bug. She went shoing with Jessica for bridesmaid and wedding dresses.Sara went into the changing room, thats funny, thought Sara,have I gained some weight? Her stomach had grown a little and was slighty round not enough to notice unless you looked closely. But she didn't think much of it it wasn't much. She put on the pale blue dress Jessica had picked and left the changing room
"what do you think, Jess?" then she felt faint and collapsed.
When she awoke Jessica was above her looking worried.
"Sara are you ok?"
Sara nodded and continued shopping not thinking it was that imortant until they went for make up in the chemists. Until just before they went to pay Jessica said "Just a minute i need some pads."
Sara suddenly relised she was three weeks late. The sickness, the fainting, the sight weight gain. It made sense. Shit I think I'm pregnant. I'm only 15. Imhoteps child she had known about getting pregnant but he was dead she had thought it was impossible. Then Sara got lucky.Jessica handed her money "can you pay for this while I go next door to get a newspaper." Sara added a pregnancy test to the basket and went to the counter she was given a funny look when she bought pads and a pregnancy test but luckily wasn't questioned aough the middle aged woman at the counter gave her a look that clearly said whore.
She hid the test in her bag and met Jessica with the rest of the things they bought. Deciding they were going home Sara and Jessica held a mini fashion parade to the famiy knowing she needed to be alone for the test and wouldn't be left alone.
"I've got a bit of a headache I'm going to go upstairs." Sara announced and noone thought twice.
Going straight to the bathroom Sara did the test waiting for the results was the longest three minutes of her life. Sitting on her bathroom floor she waited. In a way she wanted Imhoteps child she loved him. But not like this. If there was a way she could be his queen then she would gladly have his child but her family would never forgive her they could do maths they would see the childs brown skin and know the ony time in egypt she was away was when she was with Imhotep.
Two blue lines.
Sara was pregnant.
She burst into tears unable to keep control tears for her seperation from Imhotep, tears for her family who would hate her, tears for her poor child who only a month in her womb was already hated. Well would be if they knew it existed. Her family would make her get rid of it she couldn't let that happen. But she didn't know what she would do. She curled up into a ball one hnd on her stomach the pregnancy test clutched tightly in the other.

Alex was going to marry a wonderful girl he loved and was about to get a job working to find the dragon emeror in china but was worried for his sister noone had noticed but since they left eygpt Sara had been sad he could see it in her eyes. He passed her room and heard her crying hoping to catch her unaware he didn't knock hoping to see exactly what was going on so he could help her. He opened the door and saw her through the oen bathroom door. She was curled up in fetal positon one hand on her stomach the other wraped tightly around something.
"Sara? are you alright?" Startled Sara hid what was ever in her hand in the bin and wiped the tears away.
"Nothing"
"Of course somethings wrong come on sit down on your bed and talk to me."
As soon as she sat down Sara realised her mistake as Alex reached into the bin "no" she pleaded but Alex told her
"since you won't tell me whats wrong i'll find out myself."
He took one look at the test and "holy shit Sara, I take it from your reaction this means it's positive" Sara nodded tearfully her hand flew protectively in front of her stomach.
"Is it...his?" Alex asked dreading the answer.
Sara nodded again "It must be there been noone else."
"Did he rape you?" It was terrible but he sort of hoped he had.
"No, he didn't force me any time,"
"Anytime? how many times did you shag him?" he was practically yelling.
Sara mumbled something incoherant tears driing down her face. Alex felt guilty and put his arms around her as she shook sobbing.
"I'm sorry Sara,look I won't tell anyone.You can hide it for a bit. I'll take you somewhere to get rid of it noone else needs to know."
Sara pushed him away both hands firmly over her belly.
"No, Alex I can't kill it it's my child."
Alex looked sadly at his sister.
"Sara I love you and will do anything to help you but theres no way mum and dad will let you keep this baby. I'm offering you way out where they're not upset with you."
"And you, you must think i'm disgusting."
Sara had slept with their enemy. She was carrying his child. She loved him. He could see it in her eyes.
"Why did you break your promise to me?" he asked
"When your with Jessica does anything else matter?" Sara demanded.
"No" Alex replied
"Thats what I felt with Imhotep."
Sara really loved Imhotep Alex knew they should be together.
"Pack a suitcase Sara and leave tonight I'll give you money.Go to Imhotep."
Sara quickly packed a weeks worth of clothes and her books. She didn't need much.
Alex gave her a hundred pounds which was alot in those days.
"Thank you Alex and I'm sorry. I'm so sorry." Sara whispered hugging her brother.
"Don't be Sara it's alright."
"Your going to be in so much trouble because of me."
"It's alright I believe I'm doing the right thing."
Sara snuck out the back. That night she got on a boat to eygpt one week and she'd be with Imhotep.

Alex lay in bed he hoped everything worked out for his sister. He awoke at seven o'clock to his mother screaming.
He only hoped Sara had already left. Once she was with Imhotep there was nothing their parents could do.