Congratulations on the coming baby.
First I want to say that it is possible to make this work. I am married to Maca and we and GG decided to have a baby-she is biologically mine and GG's, but because in our family home there was already my daughter, Maca's son and maca and I's son who all call maca and I mom and dad, we opted for having her call maca dad and GG by his name. It's a technical detail that worked out well for ensuring a sense of security for all four children.
BUT-I also want to caution you to look into legalities before you suddenly come out in a big way.
There have been issues with people who come out as poly and then have family members drag them through court over custody of the kids.
This isn't to say that they lose their children. That generally won't happen unless someone can prove neglect, abuse or mental incapacity of the parent.
BUT dealing with it is a big big emotional nightmare.
So check into it.
In no way am I trying to bring a gloom and doom attitude on your situation. It can be a WONDERFUL situation for all of you. But just be sure that you are cautious in ensuring that you know your rights.
You might call and ask children's services what their take on it is-anonymously of course.
Good luck and again-congratulations!
"Love As Thou Wilt"