How can we make this work?

my fiance is having a hard time when we have my children on the weekends. 3 of my older sons live w thier dad and my youngest son lives w/ my fiance and i and ebverything is fine w/ my youngest son they already have bonded and its great but we have my older sons ages 12,10and 5 ever other weekend and my 12 and 5 year old are kind of difficult to deal w/ my oldest talks back alot and is very outspoken and my 5 year old (who by the way is on nedication for epilepsy which affects his behavior) talks back as well and has said your not my dad and had outbursts of anger. and my fiance has a hard time dealing w/ it he has 3 sons of his own and compares my kids behavior to theirs he says he cant handle their behavior but loves me and wants to try so how can we make this work?