It is the raisen d'etre of a 16 year old to make their parent feel guilty.
She has found your soft spot, and is doing what most teenage (especially girls) do.
She certainly wont have had a "normal" upbringing. There are many things she may have missed out on, but, likewise, there are many things she has gleaned from having this unique family of hers.
The only thing you could possibly say in response is, that for as much as she may feel that she has missed out on normal sibling stuff, she has had far more of your attention than she would have had, had she had a sibling to share your already shortened time with.
It must be awful that she is saying these things to you, but, you certainly dont have anything to feel bad about.
Im not at the teenage angst stage with my two so I dont honestly know how to advise you to deal with it, except that, it seems like a teenage issue rather than any other kind of issue iykwim?
I must say, if that is all she has to 'complain' about, then she isnt particularly hard up, is she? Sounds like you have done a good job in what must have been a tough situation.