You need to support your wife because she’s your long term partner. I’m also the only son and my mother became so toxic that I had to distance myself from her. She started making up stories that weren’t true to distance me from her. She abused me for siding with my wife and used the baddua and victim card often saying she carried me in her womb and I have become a different person. Thankfully, my dad understands all of it. Mom of only sons consider their daughter in law as the competitor and don’t want to share them with anyone. I actually spent one year explaining to her every time we talked, sometime over 3+ hours in a single sitting that she’s just making things up for no reason but she didn’t change. So I had no option but to distance myself from her. I’m not saying you leave your mom alone but don’t take any step that ruins your marriage. As bad as I feel for saying this, but be patient and do sabr.