I’m sorry for your loss. But I do think you are being harsh, unless you asked them to come, they said they would and then never showed up.
A lot of employers will only give paid leave for funerals of immediate family. Maybe they couldn’t afford to take any unpaid leave off work. Maybe their employer wouldn’t let them take time off work for their “son’s girlfriends mothers funeral”. Maybe they’re worried about being pushed out at work so they didn’t want to take time off.
You said your brothers in laws attended - is your brother married or is your SIL a girlfriend? How many times had her parents met his parents? Maybe they were just closer than you realised. Or maybe your SIL was really close to your mum, so her parents were supporting her rather than your brother.
if my in laws turned up to a member of my families funeral, I’d be pissed off because I know it would be performative bullshit. They didn’t even ask if I or the kids were ok, or say sorry for your loss, after we mentioned my grandad had very suddenly passed away and they knew we were all incredibly close to them and that our kids would have been devastated. My parents have tried so hard with them, but I think they’ve only met them 6 times in the 26 years we have been together (married 22) and they made it very clear they look down on my parents. They disapproved because they ran a pub and they didn’t like seeing the kids running around in it.
Long winded way of saying I wouldn’t automatically expect in laws to show up for funerals without explicitly being asked.
If you’re seriously going to use this as a reason to cut off contact with them now, you need to tell your partner this, as he has the right to know that if he marries you, his parents won’t be in his life as much because if your choices. He may well choose to leave rather than pick between you over a perceived wrong doing. So be very careful in what you choose to do - maybe wait until your grief has settled. Also, be honest with yourself... If your boyfriend’s brother‘s girlfriend’s mum died, would you even know it had happened, and would you take unpaid leave off work to go and attend her funeral?