Will i tell FIANCE before WEDDING that baby is HIS BROTHERS?

I am due to be married in 12 days. My fiance is 58 and his wife died 2yrs ago. They didnt have kids but he always wanted one. He fell for me when i had to go around and scrub his house while workign with an agency. Hes a wealthy man and i'm already set up in this 8 bed mansion and now an agency is cleaning FOR ME. So life is good to me. However, I've always been highly sexed. My husband can satisfy me somewhat, but i'm an everyday kinda gal. Hes more a once or twice per week type and less. Anyway, i'm ashamed to admit, at the start of our realtionship i strayed. But i was not careful and had sex with his brother about a dozen times and also with a nephew of theres. I know, i know, NO LECTURES please. Anyhow i'm 6 months pregant. My fiance thinks i'm up the pole with his kid and is behaving like a doting dad to be. However, i had sex with both of those without protecction ....so i cannot be CERTAIN that this childs not belonging to my fiances cousin or brother. There was also another ex but i used condoms with him so hes off the list. What will i do? this is an awful mess and stressful for me as his nephew is dark skinned and my husband fair with blue eyes. I'm also a blond and fair skinned. WTF will I do? My parents say shut my mouth and hope for best that once i'm married i've more rights.