There's no point in me stating my viewpoint, Calina has already put it far more elloquently and succinctly than I ever could. (I'm a self-confessed waffler)
I just wanted to add 2 examples of what I see as terrible parenting and complete lack of consistency
Mil is terribly passive agressive and well accomplished in martyrdom. If H ever stropped or agued with her she would just let him, no discipline, no telling-off, no consequences. She would just stand there and look sad/upset/hurt. Consequently he would get halfway to school and have to turn around and walk all the way home to apologise because he felt so terribly guilty.
Fil is a bully, plain and simple. He acts all easy going, happy-go-lucky then loses his temper in a flash without any warning. He once used a belt on H when he found him taking money from Mil's purse. Only afterwards did he ask for an explanation. Turned out H was taking it to pay off the bully who was beating the crap out of him every day. (Interestingly they didn't feel the need to do anything about the bullying)
Result - H has next to no social skills, no sense of boundaries and feels the need to win every argument by either sulking, shouting as loud as possible or both.
DN (4) is smacked by BIL. Not often luckily because she really is a lovely natured child. Unfortunately her one failing is that she has inherited her father's short temper. They were called into Nursery last week because she punched another child.
The nature v nuture argument could go on forever with both these examples but surely it's impossible to ignore the obvious message.