So, my parents have split up with my mum keeping us (sister and I) most of the time and us seeing our dad every other weekend (assuming that its possible). Only recently when it has been made clear on both ends that we have a choice to go to our dad's my mum has also started saying things like "well I suppose I'll have to take you to your dad's" in a rather derogatory tone, making comparisons between her and him which boil down to her asking "who's better/who do you like more?", and just other kinda rude but not offensive in any way things. Of course this makes me INCREDIBLY uncomfortable and I try my best to avoid answering the question and/or talking to her for the rest of the day when it's brought up because I just think that the way that she puts her in a decent situation, my dad in a pretty bad one, and me in an awful one is ludicrously scummy. 

To provide some context: I don't know the exact nature of why they split up, my dad has recently had a baby with another woman (I personally am ok with it), and the frequent of us seeing our dad seems to be dropping