It sounds like OP still lives at home, so nope, they're not all grown women. Depending on what happened, it would be impossible to have a relationship with OP while she's under their roof.

I'm not making assumptions, I'm pointing out what I'd think was obvious--OP adored her one sister and it sounds like that was mutual. Barring mental illness or an abusive new husband, it is very unlikely (to say the least) that the relationship changed without reason. Her sister probably didn't wake up one morning and decide to be a terrible person.

The other possibility--which did seem mean to bring up--is that OP was a spoiled teen who took small slights (that weren't necessarily even meant to be slights--calling out the sister when she said she'd be going wedding dress shopping with her real mom... The sister could just as easily have been embarrassed and conflicted as intentionally cruel. Teens are notoriously bad at reading situations and people--they frequently mislabel fear as anger, for instance--and thus unless OP is the world's most discerning teen, she's a very unreliable narrator here.) But that didn't seem kind to point out, as scientific as it is, because I understand she's hurt here, even as she seems to be exclusively aware of her own hurt and completely unwilling to consider perhaps they were hurt too.

I think it's short sighted and perhaps outright ignorant to think OP can understand her relationship (or lack thereof) with her sisters without understanding their relationship with her father. OPs mother stole their father when they were young (the eldest was merely ten when OP was born). OP shouldn't have to apologize to her sisters for having been born, but those young women do deserve an apology from their father, and if she knew he was married, OPs mother as well. (Also, I'm aware women do not steal men from families, but ten year olds sure as hell aren't generally.)

The situation is a mess and OP is being a petulant child about it, whining why should she have to apologize when she was the only one hurt. But a) she obviously wasnt the only one hurt, and b) her sisters aren't demanding (or even asking) for an apology. Rather, her father is demanding one. Thus, what the sisters did doesn't even matter--she needs to confront her father about this, not relive all the ways two young women hurt her many, many years ago all while not even giving a thought to how she might have contributed to the situation.