I am so sorry. Losing your Mum is so unbearably hard.
When it comes to funerals, there are very strong cultural differences, broadly 2 camps.
There are some people who feel that attendance at the funeral is primarily about the direct relationship to the deceased. In this group people attend the funerals of people they have known, or in order to directly support a very close relationship e.g. partner
And in the other camp, people that feel attendance is about respect to the family and community of the deceased. So in this group people will attend funerals of people they've never met e.g the parent of a friend or work colleague. Almost everyone who lives in the local area will attend etc.
It can feel very hurtful when the expectations don't align.
No-one's in laws attended my parents' funerals, and I would actually have found it intrusive if they had. There were people I'd never met before so it's not about needing to know them myself. It was great talking to former work colleagues about memories of my Dad etc.
I think you and your in laws have different expectations and as you are feeling so vulnerable, its hit you hard. But I don't think you should write them off because they don't match your way of doing things 