There are people that are quite lucky with their families, they have nice relationships with each close member and in general once they form their own family, this keeps being the same. It can also happen that the family of their partner is equally welcoming and they are able to become friends. However, sometimes few problems arise because of the families each partner “carries” with them.
In my own experience I can only say I have been one of the lucky ones, the only person with whom I clash is my mother, and I have started to think it is just because we are mother and daughter, I know plenty of female friends who have these silly fights or arguments with their mothers because sometimes they are just too much, I try to avoid these situations with her since I know in which cases she gets annoyed at me, so in the end I do not really have much problems, she is a wonderful mother, and I am very grateful to have her in my life, even if we clash sometimes.
Iliyan is a very peaceful person, and I saw that in his whole family, they are caring and calm at all times, they have given me loads of love and I can only be grateful for that. Because of his calm nature, neither he or my family experienced clashes of any kind, he has always been helpful and thoughtful towards them and in general my family has been kind to him — there were only two or three instances of my family being rather harsh towards him, for, quite frankly, stupid reasons. Yet my fiancé just tried to be better for my sake, and for that I am a million times thankful.
A friend of mine told me about the mother of her partner, that she was rather controlling of him and would call at very inappropriate times to talk for hours with him. It bothered her that the person whom she loves is being pushed in every direction at the whims of his mother. This situation even caused a bit of a bump for them. She came to talk it with me and my only advice was to discuss it honestly with him, explain what bothered her, but by no means to start attacking his mother or instructing him to tell her off, that is something that he should decide whether to do or not.
In general that is the whole trick to these situations, when one of your family members, or of their family members, is a problem for you as a couple, individual or family, you should always talk it with them first, in no way get all angry and acuse the other person of being the worst, it is the family of the person whom you love, this would only hurt your significant other. They should be the ones taking action (if any), if the problem persists and your partner does nothing about it, then you can consider having another serious talk with them or think of other solutions for yourself and the family that you have formed with your beloved, putting your family first is natural and not bad.
In conclusion, try your best to resolve things peacefully and in a civil manner with your beloved and if needed, with the family that poses a problem. Clear and firm communication in most cases is the key. Nearly all clashes are resolved like that, if it is more serious and it poses an emotional stress, then one might consider limiting the contact with said family, or member of either of the two families.
All and all, families are crazy and wonderful, treasure them!