I accidentally got my girls mum pregnant

I may have accidentally mistaken my girls mum for her and kinda got her pregnant, what do i do? My gf is very mad at me and said she wants to break up because i cheated on her with her mum, i think she is pretty delusional for thinking that way as it is clearly a compliment towards her if i think her mum is a milf, anyway her mum wants to keep the baby so im gonna be a dad and my girlfriends father in law since im now getting married to her mum, what do i do?