I kind of resent my father for using me to try and calm the raging beast. At this point, he made his bed , he can sleep on it.
It's never fun to be put in the position of having to fix someone else's issues, so I get where you're coming from. Many members here (including me) have enabling and co-dependent fathers. It's very frustrating.
My husband pointed out something to me once: it's so much easier for people like my father when the person with BPD (in this case, my mother) focuses her rages on someone else... . anyone else... . as long as they're not focused on him. It drives me crazy at times, but in the end, I know that he just cannot stand up to her. He can't. He's so afraid of losing her that he can't rock the boat. It's so much easier, then, to keep her focused on someone else (me, my SIL, the neighbors, a Starbucks barista, Anderson Cooper on tv, whoever happens to be convenient).
Can you have a relationship with your dad that's separate from the relationship you have with your mother? When he calls, do you have a way of keeping the conversations with him (or your mother) from going into discussion about your brother and SIL or other topics you're not comfortable with?