Backstory:
My brother is 2 yrs older than me. We grew up privileged, in a nice area with very lovely parents, wanted for nothing. He was always very academic, always had top marks in science and maths, played football and was quite good, relatively attractive so no problems with finding girlfriends etc. I was more arty, loved writing and performing, good singer etc. We went to different schools (single sex) yet he was always in competition with me. I was still good at school and always had really good but not always top marks which he used to rub in my face, calling me thick etc. He just has a very nasty streak. If he could hurt me, he would. As he's very clever, he was very good at doing this subtlety and wearing me down over many years. As an example, when I would dress up for an occasion, he would side eye me and snort/laugh then just turn his back on me. I would call him out all the time but again, because it was always subtle, he could deny it or make it seem like I was overreacting. But this was constant.
When he went to uni, my life was much better, I had freedom and he was gone. He was quite nasty to my DM too, he would ignore her calls and texts all the time, told all his mates that she was really annoying and pathetic, but she loves him so when he did bother with her, she was almost grateful. He studied an academic subject at a very prestigious uni and did very well, went on to get his PHD and now has a successful, well paid career. I studied an arty subject at uni (still got into a good one!) and went on to be a wedding and events planner for a number of years. He mocked and belittled my course and later, my career whenever I saw him. He would always put me down wherever he could and caused issues between me and my friends/boyfriends. He told my first long term boyfriend that I was cheating on him and had 'proof' which was completely fabricated.
Now I always have been a very confident person, I call out shitty behaviour when I witness it, I've always had lots of friends and a great social life, I've taken lots of big risks and made some crazy choices but I've had a great life off the back of it. He has always played it safe, only had a couple of friends who he isn't close to and treats his wife appallingly.
So now, I have a 2yr old DS with my DH. DB, when I first had DS, started trying to plant the idea in DHs head that DS wasn't his. Saying things like "baby doesn't look anything like Bianca, or you. He actually looks a lot like X, don't you think?"
Now DH isn't stupid and basically told DB to do one.
We bought our first house recently and my parents very generously gave us half the deposit. We are a low earning household despite both DH and I working full time so we would never have been able to save up that money on our own. DB went mad and demanded the same money from our parents even though he didn't need it. My parents also help us with childcare and he has now asked my parents to give him the same amount they save us in childcare fees to make things 'fair'.
Very recently, there was a family get together for my parents 'big' wedding anniversary so obviously DH, DS and I were there.
DB has had nothing to do with DS before but he was actually great with him. DS loved his uncle and they played together the whole time almost. I was hesitant but thought that it was good that they have a good relationship even though I don't have one with DB. At the end of the day, my DS told me I was stupid and thick. He's never said this before. I suspect my DB was saying this to him to stir some shit. When I told DS off for things (playing with food etc) DB would undermine me and do silly things like stick his tongue out at me, so obviously DS copied.
It recently came out that DB is suffering with depression and is seeing a therapist and taking medication, apparently he was toying with the idea of suicide. My DM has approached me and told me I need to be the bigger person and make an effort with my DB in order to help him.
Tbh, I don't want to. I have nothing to do with him unless I absolutely have to, and I don't want that to change. I don't want him in my life. I don't really want him around DS. DH doesn't like him either. My DM has practically begged me as she can't face losing her son which I fully understand.
I just don't know what to do.
(Have changed some details as anyone who knows me IRL would recognise me from this I think)