They do it because they CAN!!!! If the parents did their jobs correctly - and showed them how to be responsible adults - they wouldn't be in this situation now would they?
My daughter lived with my ex-husband his next wife after she graduated high school and before she went off to college. She had to pay rent. My ex and his wife (whom I really like) put the money into a savings account for her that she didn't know about. She left college for a year and went back to their home...where she paid rent. She had to work at Jack in the Box...she went back to school and moved back with them again after graduating...while she was searching for a job...again paying rent. She got a job and asked to stay - they said fine - rent is to be paid. She now has enough money to buy her own home (down payment) with furniture or a new car....her choice.
Any way - sorry - I digressed...If **I** were parents? I would tell them they have 30 days to find a job if not -you are out. This means showing proof they are sending resumes for positions they are qualified for and such. If that doesn't happen? HASTA LA VISTA, baby.
If it was my brother? I would kick his butt in gear.
Since it's not your business - unfortunately - you can't tell your parents or in-laws what to do...IF they ask or complain about their situation? TELL THEM YOUR OPINION...
I would stop dating your brother if I found out he still lived with his parents at the age of 35....
to be honest - I don't see this situation changing as the parents aren't stepping up...if he is verbally abusing his mother? I would report him to Adult Protection Services...yeah - he's my brother - but you do NOT disrespect your mother. If I was around when he did it? I would correct him on the spot. If he didn't like it? I would tell him to get F&&K over it and grow the F&&K up....