Maybe its seeing this through my mother in law’s and my husband’s eyes, but I tend to agree with Christi. Maybe your birthmom is being selfish and fucked up. But maybe she is trying to protect your sister from something she thinks will be harmful. It certainly was not easy for my husband to have a sister dropped in his lap, and, I suspect if my mother in law had been LESS selfish about the situation, my sister in law would have been kept at arms length from the family until the boys were old enough and stable enough (their sister came into their lives when they were eleven and twelve and their mother was divorcing their father - not the best timing - and the combination screwed them both over for a long time). I know this isn’t your situation, sounds like you sister is a young adult, but there may be background here you are unaware of.
It sounds to me like birthmom is very conflicted. She wants you to know everyone, she doesn’t want to have to re-experience the guilt and shame she went through 35 years ago. She wants honesty, but to have it she needs to admit to her daughter (and likely other relatives) that she has been lying her whole life. She wants to be able to give you some of what she couldn’t give you then, but she doesn’t want to fuck up again. She wants you close, but doesn’t want to risk losing you all over so pushes you away. She doesn’t want to risk hurting herself or her family if you turn out to be a flake, yet she really wants a relationship with you. She’s spent fifteen years developing reunion fantasies in her head (and probably she’s been doing it for your whole life), and now presented with reality she needs to start facing the reality - when is never as pleasant as the fantasies. Given the situation, this doesn’t seem fucked up or selfish - it seems very normal. Yes, its inconsistant as hell - but I’ve been remarkably inconsistant in my feelings about many less important things in my life. If her relationship with her daughter is rocky, or her daughter is less than stable, wanting to put off telling her until she sees how you and your brothers work things out may be not selfish at all, but very wise.
On the other hand, she may just be selfish and fucked up.
I’m hoping the benefit of the doubt is warrented and patience and communication pay off for you elmwood.