It sounds like it could be a very unhealthy
situation to raise a child, she may have had the best intentions and may be seriously only thinking about the very real situation of raising a child in a poly household. Good intentions will pave a road straight to hell if she isn't smart enough to care about how that would affect you or the child.
But to be honest, the main problems facing poly child rearing these days is shielding them from affliction courtesy of society and enforced beliefs.
Besides parenting , the legal challenges that will be faced present complicated problems. But they are problems that can and will be dealt with. The greatest distinction if shit hits the fan, by that I mean it does not have to be a distinction that is apparent unless there are problems, but there is a difference between biological parents and non-biological. Poly-step-like parents or whatever you wish to call any other parental figure who is neither of the two biological parents.
So unless you are planning on willfully surrending your parental rights to your SO's spouse, it sounds like a fucked up thing to do if you aren't OK with it.
these sorts of complications, especially the legal ones, are exactly why poly families will not have their withheld Constitutional rights granted and protected until the tale end of dispensation as opposed to the beginning.
The worst part is, each of US as people would know beyond any reasonable doubt what the right thing to do is if we were practiced with honesty. It is failure to be honest when were are hurt, or being practiced at lying to ourselves that we justify our actions to be the right thing to do, which of course does not make it true in reality, no matter how much we believe it.
Very, very rarely would the courts of law be even needed if people were practiced and fluent with honesty.Most if not All of the controversy and grey areas that fill the news of the legal world would be distinctly black and white if all parties involved refrained from exaggerating and justifying their dishonest behavior