Not sure why you are feeling stupid? For getting involved in the drama? It happens.
First you have your AW with erratic calls, then you have the exSIL and her screenshots and then you have the Brother.
It sounds like a bad soap opera but I know, it's not, it's your experience.
Perhaps taking time away from all of them is key here? It's hard to see all the facts when you are tied up in the drama. Again, it happens, life with an alcoholic comes with built in drama.