I hasten to add that this is my understanding of AEPhi's policy, and that I do not agree - I feel that adoptive and step relations should be treated as legacies, and I applaud the GLOs that have formal policies to this effect.
I think the idea is that if a woman were adopted late in her childhood, when her personality and her values were pretty much set, and her adoptive parent(s) hadn't really had much influence in her life, the chapter would not have to treat her as a legacy.
But if a woman had been adopted early in her childhood, or if the adoptive parent(s) had been a big part of her life, she should be treated as a legacy, and I think most chapters would do so even without nationals twisting their arm. If I discover I cannot have children of my own, I fully intend to adopt, and woe betide the AEPhi chapter that doesn't treat my little girl as a legacy!
And really, what's the big deal? PNMs aren't going to be advertising their status as adoptive or stepdaughters during rush. You invite her back, and then if it's decided that she wouldn't fit well with the chapter, then you just don't invite her to pref, just as you would do with any legacy. It's not such a big deal.