Complaining to your mom that your sister isn't planning around this semi-irrational fear and making you look like an ass is being a bitch.

She already offered a fantastic solution.

The sister could stay away from dark alleys, strange men, and stay sober if she is so worried about it.

Instead she complained to her family that her sister wouldn't rearrange her entire party of 60 people for her, and yes, moving reservations and calling 60 people to change the time is a big deal, even if it's only an hour or two.
It's hard enough planning to get together with a group of three or more.

The sister may have a fear but she needs to get over it somewhat.
There is cautious and overly cautious and she is very much overly cautious.

I'm sure there are gonna be several people with a decent fear of heights at this party, too.
Should she move it to a park for them? No.
And if she has an anxiety issues, you don't get through it by getting it catered to.