After my first "WOW" reaction... I tried to calm myself down and look at it objectively. Having done that, why in the world is this baby shower anticipating 50-60 in attendance?! That is a LOT of people. WAYYYY too many. Most baby showers (and I have been to my fair share) are much more intimate than that. 30 attendees at the MOST. If you sisters want to give her a shower, then do so and have it for family (and maybe if there are a few super close friends that you practically consider family). Then one of her friends (I'm assuming that a good number of those 50-60 people are friends and not family), or a couple of them together, can throw her another shower for friends. They can arrange that however they wish to (and maybe your sister can suggest they do this in June so her moving friend can attend?).
You guys have made major plans around this event and she should be grateful for your generosity in doing all this for her and her baby. Unfortunately, it sounds like some hcg (hormones!!) is affecting her emotions and she is not being her normal considerate self. Pointing that out to her, however, isn't likely to help.... it will just make her defensive. I'm sure her anger or whatever is perfectly legitimate in HER mind. Telling her it is pregnancy hormones isn't going to change that. :(
Offer to make the shower smaller and limited to family, so that her friends can throw her one as well... in June. Be gracious and tell her how you are so very sorry that what you were planning isn't what she wanted, so in order to accommodate her wishes, you will limit your shower to family, so that her friends who may know better what she wants will be able to give that to her.
I'm betting that if you just change what you are offering her, that she will come around... Pregnant ladies can be worse than Bridezillas..... they should call it Pregzilla!! LOL