If you're talking about the "Family" list on the left margin of most FB profile pages, here's what I understand, from accidental personal experience:
If your SD and her Mom have already confirmed each other as mother/daughter, then their names and relationships are listed in blue on their "Family" lists. (Blue = people can click on the name and call up however much of your family member's profile page is public). If you accept SD as your daughter, her name will continue to be listed on her Mom's "Family" list, under "daughter", but it will stop being blue and will only be blue, on your "Family" list. Worse, SD's Mom's name will disappear from SD's "Family" list altogether and be replaced with yours.
So, if you're friendly with her Mom, why not warn her that you're going to accept SD's request and that she should send SD another family request afterward and SD should accept. Then, SD will be listed on your family site, but not in blue; she will still be listed in blue on her Mom's list; and on SD's page, her actual Mom will be listed as her Mom. Then you can add your other kids and not feel like you're leaving out SD. Your other kids will be in blue and she won't, but they'll all be on there.
FWIW, I think it's absolutely ridiculous, in this day and age, that a technology like FB offers no options to recognize step- family members and that a person can't confirm two parents of the same gender.
I also think it's interesting that, thanks to FB, we have all these new "manners" issues to consider. I know some bio parents are offended if step-parents post any photos of their kids, or even talk much about their kids. Personally, I'd be insulted if my kids' step-mom only talked about her bio children and didn't talk about and post photos of mine. Also, I have a young step-son who lives with us, who's listed as my kid and (until his Mom joined FB) he listed me as his mother. But I have an older step-son, who was in college when DH and I married. I'm FB friends with him, but I haven't listed him as family. I worry that he's insulted by it. But I'm very conscious that I did not do any part of raising him and I don't want to offend his mother, who's also his FB friend, by implying that I've been any sort of "parent" to him.