Partner’s family helped his brother with a house, but not us—I'm struggling to reconcile this

"Thou shalt not covet" is the tenth commandment, prohibiting the desire for what belongs to others. It's about controlling one's heart's desires and not allowing them to lead to sin. The commandment emphasizes the importance of contentment with what one has and not being driven by envy or greed.

Unlikely.

I struggle to see why you are so invested in somebody else’s parents - they’re not yours - and what they do with their money. I can understand why your partner would be hurt, but I’m not sure why you are.

My mother gave money to both my siblings to help them buy houses, but not me. She thought I was sorted because I was living with a partner who owned a house (she didn’t know I was trying to leave a violent relationship with a child and had to wait until I earned enough to get out and could have done with that help more than the others, because I didn’t share that with her). When I was a child both of my siblings got more materially than I did - in later years I realised it was because I always had lots of friends but they didn’t and were bullied and so I can now understand.
Rightly or wrongly your partners parents might think he’s sorted because he has you and the brother needed help more now, and they might well even things out in the will.

But none of that matters really - the reality is as many here have said, most people have to make their own way without help from parents. You guys over reached with this house and are now in difficulty - that’s not on your partners parents, it’s on you guys. From your perspective - I’d be considering how much you are giving to this relationship and how much you are getting. What is your partner doing to step up here to support you - apart from wanting to be a kept man? I’d focus on that, rather than the financial help his parents are giving their other son, which should not be of concern to you personally.