My brother has a manipulative wife, which has essentially turned him against our mother. I want to keep the story short and won't go into too much detail. Essentially, his wife has jealousy and insecurity problems (diagnosed by professionals) and, from the beginning of their relationship she has been jealous (as acknowledged by himself) of our very close relationship (our mother, my brother and myself have always been very close and relied on each other).
At the start of the relationship, my brother was able to keep his boundaries, but the situation has gotten worse due to some recent developments and my brother has started blaming our mother for all his problems. Examples of her manipulative behaviour include threatening my brother with suicide (when my brother openly proposed separation as an option) and blatantly lying about how my mother had behaved.
This has led to him thinking that it is his relationship with our mom that is toxic, and whenever it should be noticed that my brother does not look happy, quite the contrary, he looks rather depressed, and I feel that he is just unable to emotionally accept that his wife has tried to destroy our relationship from the beginning. It also looks that he is trying to deny his own feelings by trying to believe that the problem is our mother. I have tried to address the situation, but my opinion has been discarded as "I am myself manipulated by our mom, and thus I am not objective". I have tried saying that I am here to support him, but he does not want to talk or share his feelings anymore.
For me, this is a painful situation. My mother lives alone (father died) and I live abroad with my partner, while my brother lives close to my mom, in the same town. Some weekends I visit my home town and I stay in mom's house. Whenever I come my brother always invites me to go visit them (he and his wife). I have noticed that he has started inviting me only when his wife's family is there. He does not invite our mother.
What I Want
I am smart enough to understand that this is a no-win situation (several answers in this site back this up), you can't essentially convince someone so emotionally involved with someone into thinking rationally about his relationship and boundaries.
However, I want to say no. No to meeting with him and his wife's family while my mom is left aside. I want to do it in a non-confrontational way. I have previously used excuses (I have work to do, I am not feeling well...), but now I want to be sincere and express my feelings to him: This is an injustice for our mom, and I want to stay with her. I am open to talking to him, helping him, and supporting him with whatever he needs, but I will not participate in leaving our mom alone while the rest of the family spends time together. I am willing to spend time with him, but not like this and not leaving aside our mom. I feel like doing so would be accepting and supporting an injustice.
How do I do it? How can I be sincere while still conveying that I am not rejecting him or want to destroy our relationship?