What is a baby shower these days?

Ever since I can remember a baby shower has been an event for the women close to the mom-to-be to get together and gift things for the baby. My sister is pregnant right now and due in July. She will be having her baby shower in two weeks. She sent out all of her invitations to about 30 women she wanted to invite. Now she's getting RSVP's in the mail from individuals confirming that they will be attending along with their mothers, husbands, sons, etc. I think this is ridiculous that these people are inviting others to my sisters party. She has only planned on the women she invited and can only afford to host the party for those few. Now I feel bad because she's getting stressed about how she's going to afford all of the food and have room for everyone. She doesn't know how to tell them they can't bring the others. Shes a very sweet girl and would feel bad about turning people away. I just feel bad for her and the predicament that she's in now. Why are these people inviting others and why are they men? I'm just baffled.