parenting children other than your own

IF someone out there could give me some advise that would be amazing. Here's the deal: I have two children of my own and doing obviously the best I can to raise them. I am now involved in a very serious relationship that I see myself being in for the rest of my life (yes I am aware that is a long time and bold statement) with a man who also has two children. I have my kids 100% of the time and he has his kids 50% of the time. When his kids are here my daughter (age 4) acts out very much... I know a lot of it has to do with having to share attention but another huge part is that when she approached his kids they always have a rude tone and bad attitude towards her which in turn causes her to act even more negatively towards them. I am not sure what I am suppose to do. I discipline my daughter accordingly to her actions but his kids are never spoken to about having a negative attitude. I am not sure how I am suppose to approach the situation because like any parent my companion is gets very defensive when I try to talk to him about and behavioral issues his kids might have. PLEASE give me some tips on how I should approach this, I am so tired of my kids being punished non-stop and hurt that my daughter only wants to be friends and feels like she is being turned away. I never want my child to feel unwanted especially in her own home. Any feedback is much appreciated.