Yeah I'm not getting why you go there OP. Don't visit them, and tell your partner that you are not going either. They do sound like horribly controlling and abusive dicks though. 8 years is a fairly small age gap, so don't worry about that. They are just finding anything they can to discredit you (and the relationship you have with their son.)
I know someone right now who has an emotionally abusive, controlling, narcissistic mother; his dad was with her for 10-11 years and she had 2 kids with him. 12 years ago, when the kids were about 8-10, he left her, (they never married.)
This lad (I'll call him Josh,) put up with his mother's behaviour until he was 21; she would scream in his face, tell him he is stupid, laugh at anything positive he did, refused to let him go to uni, and took almost half his monthly wage because she 'needed it' because she lost £350 in tax credits and child benefit (when he hit 18,) and she regularly hit him (and his brother.) His brother is now an aggressive and arrogant and shouty misogynist, and Josh is quiet and timid and rather fragile.
Josh met a wonderful girl who was feisty and confident and started telling him to fight back. Mommie dearest didn't like that and did everything she could to split them up. She didn't manage it and pushed them closer together. So he left his mother and moved in with his girlfriend.
A few months later, the brother left too, to join the services, and the nasty bitter little troll of a mother was then alone, and £800 down on her income. The dad of the 2 boys had been paying the mortgage on the house since he left 12 years before (the house cost £65K in the 1990's when they bought it, and only £15K was left, and the house was now worth £290K.) He also stumped up a couple of 100 a month each for the boys too.
Even though he didn't love her and didn't want to be with her, (because he realised what she was like) he still wanted to look after his sons. But he said very early on, that when both boys left, he wants his equity from that house, and will sell it. It was in his name, he had looked after her (and the boys) financially for 20 years, and so he sold the house.
Result for being a cunt? She had to get a JOB. And she lost her house. He did give her £100K though (of the equity,) towards a new house, which I thought was rather generous really.
Of course, she blames Josh and his girlfriend for everything that's happened, even though SHE is the one to blame, and as I said, her ex said he was going to sell the house when the 2 boys had left home anyway. This all happened 20 months ago, and Josh has never been happier. He is started to get a lot of confidence in himself, and knows he is not the worthless piece of shit that his mother made him think he was. Much to the old harridan's disgust, Josh left and lever looked back, and has had sod-all to do with her for all that time. He and his girlfriend never see her. Most of the wider family hate her, so it's fairly easy for them to have a relationship with them.
Only her brother, and the other son took her side. Josh still has loads of extended family. He has his dad, step-mum, step-sister, (and her family,) and also a number of aunts and uncles, and grandparents, and cousins on his mother's side AND his dad's side. Many of them confess to not liking his mother, and most have them have nothing to do with her.
Not all mothers are wonderful human beings who deserve love and respect. Some are cunts.