My DH is lovely but he doesn't stick up for himself with his family. I can't stand it as they trample all over him (not literally!) and I don't think they treat him right. They dismiss his feelings about important issues and blame him for things which aren't his fault.
He knows that I feel he should be more assertive with them but he seems to think that the options are to either be completely passive or really agressive and fall out with everyone. He doesn't see the inbetween.
Because he doesn't assertive himself with things at the start it often escalates. For example a while ago his dad asked for a lift home from somewhere and DH told his dad that he wouldn't fit in the car as it had 3 child seats in (with children in so couldn't be removed). His dad insisted though but when he couldn't actually physically fit in the car he got really angry with DH and blamed him (huh?). When his dad got home by different methods we were already there and everyone had a go at us. For once I stood up for DH and said that it wasn't our fault he didn't fit in the car and that we had told him he wouldn't and he should have just listened. That shut everyone up!
I don't normally intervene as DH likes to handle things himself but that results in us being blamed for everything and lots of resentment!