don't sign the birth certificate if you stay with her otherwise you will have to pay child support for a child that is not yours
Kick her to the curb.
Do u love her ? R u willing 2 4 give her ? can u accept that child even though it's not yours ? can u love her & that child ? you really need 2 ask yourself the really important questions in life ! Sorry but life is way 2 short & u need 2 realize what's importat in life ! if u want 2 b can u b a great dad 2 this child ? seriously think about it ! the child is innocent in all this remember that please !
id go and explain how upset you are to the other guy - make him sign the birth certificate - make her sign the lease over to you - and put your feet up and watch the fireworks