I need HELP! My brothers WIFE is cheating and pregnant! DUE DATE HELP!!!!?

Ok so my brothers wife cheated on him with 3 (AHEM) that she has told us about in the last week..and is now pregnant. The ultrasound TODAY showed that she is 5 weeks and 4 days pregnant. She had with these three other men in March and my brother in the late days of March and in April. Could my brother even be the Father? We all need to know. We dont know the due date the person that did the sonogram said that it was to early to give a due date, which i found odd, but I was there and heard it myself..anywhoo, she never had regular periods..but going by the 5 weeks and 4 days anyone have an idea on when the heck the due date and MOSTLY CONCIEPTION date is?????ANY help so appreciated..and no I dont condone her behavior, but if its my neice or nephew my brother is sticking by her.