I would talk to her about the situation and tell her you want to do something, but you aren't sure about attendance since it's kind of last minute. Were you planning to invite a ton of people and you need a place to have it? If it's not too many, I would think it would be more relaxing at your house for both you and your sister.
People usually send a gift even if they can't attend the shower, so she still should get some gifts even if the turnout is low.
As far as the food goes, I don't know if any of the guests will be Catholic, but on Fridays during Lent, we don't eat meat. So unless you have meatless mostoccoli, your Catholic guests won't have anything to eat. So some meatless option would be welcomed by those people.
M.