reader question of the week #2

hmmm...this question veers into territory that I have not yet touched on this blog. I long ago asked a friend (ahem, Whitney) to be my etiquette lady on PP but she's one busy lady and thus, hasn't gotten around to it yet. But we'll try out this question to see how it goes and I'll decide from there if we want to go down this road in the future. Which brings me to reader question of the week #2:

"My husband and I want to throw a mother's day brunch at our apartment this year. We would like to keep it to just us and parents but here's the problem... my husband's 21-year-old sister lives at home and tags along to all family gatherings because her parents don't want to her to feel left out. I'd really like our parents to get to know each other without the sister there because her parents sometimes talk down to/treat her like a baby around others. Our parents have never had time together with just us and now that we're considering starting our own family I think time alone would be helpful. Am I crazy? I don't want to cause crazy family drama but have a nice, warm brunch for our parents. I guess my main questions is am I obligated to invite siblings if they live at home? Is there etiquette on this somewhere? -Emily"