Got my wife pregnant and her sister pregnant?

ok before you judge yes im a fxxckin idiot! Im 40 my wifes 30 and her sister is 35, My wife is irish im scottish and she moved to scotland with me 8 years ago we married about 5 years ago and have 3 children youngest a year old.Her sister came for a 2 week holiday here,my wifes a nurse and does a few night shifts .i was with her sister for the night and we got talking then opened a bottle of wine then kissed BY ACCIDENT when the kids were asleep we had sex.Her sister is pregnant with my baby i know with over two months but afraid to tell my wife ,to make matters worse my wife is pregnant again with our fourth child shes about 4 weeks gone if i tell her im afraid my marraige will be over i LOVE my wife and it was drunken sex it is was all a huge mistake how do i tell her without crushing her heart ,i love her to bits and i dont want to lose her! please help