I need some advice regarding being put in the middle of my husband and his ex's drama?

A little background to start with...my husband and his ex have an 8 year old son together...his ex is the MOST impossible person I have ever met on this planet, she is unwilling to compromise about anything and the more she can do to make my husband's and my life miserable the better. I somehow became the designated picker upper of my stepson on the weekends he come to visit us, my husband works in law enforcement so he has long crazy hours...so at first, before this *witch with a b* decided it would be fun to make things impossible, I didnt mind helping my husband out, now its just to the point that if I see her face I think I'm going to puke!! Anyways my husband still wants his son to come over every so often (he is off on the weekends but his ex isnt willing to meet him on Saturday mornings, according to her it has to be done on Friday evenings) but since all the drama ensued and not to mention the fact that I am 4 months pregnant...I just dont feel like being the whipping post anymore. I also have a daughter from a previous marriage who I never ask my husband to help me with, pickup and drop off to her dad let alone I get up in the morning to take her to school when he has to get up anyways and he has never once offered to help me with her. I'm just at my whits end with this whole situation and I just dont understand why I'm am so caught in the middle of all of this when I never ever burden my husband with any drama concerning my daughter or my ex. I've tried taking to him and it always ends with both of us frusterated and not reaching any kind of conclusion. I just dont know what to do anymore so if anyone has any advice or ideas please share them with me...thanks!!!