DH and I both have brothers.
My BIL's wife is very nice in my experience but I've only met her a few times, about 5 I would say. This is because she has refused in recent years to spend any time with my DH's family and BIL goes along with it because he is apparently a doormat his loyalties are now to his wife, not his mother (as we say on 'stick up for me' threads). In fact she wrote a 9 page letter to my MIL detailing all the evils done and slights directed at her (in her own mind I am told). PILs were hurt by this but me having their first gc has taken the heat off the BIL and SIL so things seem to have eased. I'm not taking any credit btw, it just might be a factor. Perhaps the SIL was convinced there was some sort of pressure and she wanted to maintain boundaries. Oh but they eloped in some exotic location and apparently she is very high maintenance. Really this is all from DH, I hardly know her, but DH is very fair and not at all judgey so I believe him.
My brother is 23 and has been with his GF for about 18 months, she's about 19/20 ish and very sweet, just a little short of finer manners and joining in with family stuff like the washing up at Christmas and engaging in conversation with the oldies. She's just a bit sheltered I think, always had things done for her and is used to quite a hausfrau sort of mother. She's not precious, just a bit oblivious to the tiny social graces that grease the domestic wheels. She'll get it - my parents pointed something out to my brother that she had/hadn't done, he somehow suggested things to her and the next time there was a similar opportunity she was mucking right in, no issues whatsoever.
My MIL and FIL are fine. I don't have much in common with them, or maybe in fact I do have too much in common... FIL was a central government civil servant like me, and every couple of months he asks what grade I am. He's never told me what grade he was but I am pretty sure he ended his career where I am now. I am inversely embarrassed by this. They are Daily Mail readers to the point of being so conditioned by it that any discussion remotely influenced by current affairs comes out in a painful series of cliches lifted straight from the rabid somewhat right-of-centre lexicon of what DH calls the Daily Heil. It pains me to spend time alone with MIL as she has nothing whatsoever to say (it seems) - it is a particular sort of middle-aged woman decline into low self esteem, shyness, or something. She hovers and fusses which annoys me but she's not opinionated or bossy which would be 100 times worse (I gather, from MN...).