I always cut t-shirts before stabilizing--just cut them bigger and get rid of hems, sleeves, necks. If you aren't going to put stabilizer on yet you will get some curl on the edges, not a problem when you go to put on stabilizer--you will have to trim them down after you get it on.
an idea I'm doing this year for my siblings is to have a photo of my folks blown up and printed on fabric (at the local t-shirt printing shop, they do great work) and then use my grandmother's lace/lines to frame one for my sister and I've got some Seminole patchwork done in my parents favorite colors to frame the photo for my brother. You could do the same with the clothing--maybe crazy quilts, etc. That would make a quicker memory project for each of your siblings.