His entire family seems a little unpredictable to me so I wouldn't take their word for it and I'm sure his sister can find other support if she needs it. This once a month attempt at contact cycle is what you know about. If he is becoming more unstable you can not count on his old routines to be a gauge of his actions and unless you have spoken to him you don't really know what his actions or intentions are. I'm not trying to alarm you. I wasn't really alarmed in reading about his actions - I'm just trying to think it all the way through.
This situation just seems really precarious at the moment and the fewer ties to bind the better IMO.