I have 3 SIL, all with children similar ages as my 2. I wouldn’t say I have a relationship with them outside of anything arranged by partner. We go on planned family holidays, stay at each other’s homes when we visit. I enjoy spending time with them when I see them, I like them, the cousins all get along well but it’s not my responsibility to facilitate that relationship. I don’t make plans to see them and I don’t see them if my partner is not present. However, we see them frequently because my partner is proactive and makes an effort to maintain a close relationship with his sisters and nieces.
I do on the other hand facilitate a relationship with my own siblings and their children. I don’t do that through their partners, I do that via my own sibling, my partner does not do help me with this.
In regards to gifts we each get for own family, his sisters buy for my children (their blood relative), my partner buys for SIL children (his blood relative), I buy for my siblings children (my blood relative), my siblings buy for my children (their blood relative), my siblings partners buy for their family. No child is left out because everyone makes an effort with their own families, the problem here is your relationship with your brother.
We don’t invite cousins to birthday parties at all and visa versa, we keep parties to nursery and school friends.