Request #256

I want to say it would be classified as YA sci-fi but not like Star Trek or Star Wars-stype…more like realism sci-fi?

i read it in the late 80s/early 90s
it is set in the then-future (think 21st century?)
a girl learns her parents are hiding things from her – including an older sister she never knew about. then she starts wondering if she was adopted because nothing is making sense aout timelines and photos and whatnot.
somehow, she transports back in time to when her parents were younger/in school? or maybe when her older sister was in high school?
i recall that she (main protagonist) goes to school with her past-family person and gets into trouble about wearing trousers (girls having not been allowed to wear pants in school until like the 70s?)

the girl ends up going back to her timeline in the end and has a better understanding of her mom/family – apparently her parents froze embryos to fund their future wealthy lifestyle or something like that

i know it’s a super longshot, but i’m hoping you can help me. this book has been haunting my dreams lately – like ‘you need to find this book’-style.