Maybe you can suggest your brother go with your dad? Tell him he'll be the only guy and you don't want him to feel awkward. My shower is today too and I know my hubs would rather die than attend.
Well hun, either just be frank with ur sister in law, or prepare urself for the backlash from ur other family! ;-) family is difficult at times, so I guess you will have to decide who u would rather have upset!?? Goodluck hun! Hope you have a great shower n get lots of what u need n want!! :-)
Its your day put your foot down. Whatever u want to come should come and everyone should just respect that.