Please help

*I talk about a lot on this site, but there are some things that I don't mention. This would usually be one of the topics I stray form however I am cought between a rock and a hard place and REALLY need some advice. So even if you read this site all the time and never comment, I am pleading with you to do so on this post.*

Here are some back facts first:
~I have a step mom. She has been my step mom for over 17 years. Shes the only woman I remember in my dads life, including my mom.
~I am helping plan my brothers and his girlfriend baby shower
~Last June my cousin was married and a fight broke out that included several poeple of the family..The major issue is between my step mom and dad vs my aunt, and older sister.

Okay onto the problem. My step mom told me that if my Aunt was invited to the babyshower she was not coming..She would send a gift, but did not even want to chance it that she may or may not show up. Well, my brother thought that was bull shit netiher one of us are taking sides and not playing favorites. So an invitation was snet out to both my stepmom and my Aunt. I have not told my step mom that my Aunt was invited because, again I am not playing favorites and I STRONGLY feel that they should set their differences aside for one afternoon and both come. (Even though my Aunt is not very close to my brother and I, and I doubt she will come, but its the point of the whole thing). Past Christmas' if there have been family differences, my stepmom would always complain that family can set aside the differences for one evening, and I think this is the same situation. So anyways yesterday my stepmom came to me and told me that she really needs to know if my Aunt was invited because she heard some stuff and REALLy is not coming if she was invited. I continued on with my playing stupid and told her I had no clue, who my brother and his girlfirend invited. This morning I talked with my brothers girlfriend, and she said that she knows for a fact that if my stepmom doesn't come to the shower that my brother would never forgive her because for starters she has been in my brothers life since he was in diapers and also because my mom is in Florida it would be very nice to at least have my stepmom there. Also again she is putting my brother and I in the middle of their arguement and that is not about to happen. So thats pretty much the story, give or take a few minor things.
My question is this...
(a)Should I speak directly with my stepmom and tell her how furious my brother and I would be with her if she did not come to the shower?
(b)Should I call my dad to see if he knows about any of this and tell him how furious my brother and I would be if my stepmom doesn't show?
(c)Should my brother and I be so furious if my stepmom doesn't show, since it is only a babyshower, even though my dad and stepmom have been active in both my babyshowers with the girls?
(d)Call my Aunt to ask her views on the whole thing?
(e)Let it go and if she comes she comes?

Please help me, the shower is next Sunday. Thanks.