****
The disturbing version is it is some sort of revenge plan, as already suspected by someone in this thread, that would be a little sick and maybe tough to deal with, my advice to the Op is, talk with your mom and set up a gameplan, you need to be on the same page because you have to be up for the case he tries to play of one against the other.****
YOu can try that and I hope it works. But if it doesn't, you can at least be comforted by the thought that it's not unusual that mothers like to one up their daughters with this kid of situation. In the book "The Art of Seduction" the author even gives it a name "triangulation." Where someone consciously plays two people off one another.
Stalker guy from the 80s befriended my whole family and my brother was friends with for several years. Then he tells how the guy double crossed him like I needed that validation.
The problme that I have with "open, honest" dialogue in general and specifically with my mother is that when she knows what I want and what I value, she knows exactly what to withold to piss me off and to disappoint me. Think back on your interaction with your mother. IF you can think of history like this, you would be better off dissing her each time you are aware of her interaction with your ex and then pretend that nothing's wrong, of course not.
From my own experience (my mother also carried on a friendship with my brother's ex wife for several years after the divorce, so she courts controvery) and the from the experience of others, parents can very much be attention whores and you should treat them as such (and not as parents deserving of your courtesy).