I realize that in my effort to quickly type up this post, I left out a few things that may bring a little more light into the situation.
1) Uncle Chatterbox had to work, and therefore couldn't attend even though he wanted to do so. Knowing this, he still bought the ingredients for the broccoli-cheese casserole that our part of the family takes to the gathering every year and helped me make it. He knew he didn't have to do this, as I would have been more than happy to have bought the stuff, but still did it. He contributed the money for the casserole, I contributed (most of) the time. Basically, he still contributed to the meal even though he wasn't going to be able to be present. I have a feeling that this was also a big reason Sis sent food home for him.
2) Bob is a class-A, absolutely shameless moocher. He will ask anyone for anything if he thinks he can get it or thinks he's entitled to it. He wastes his money then bums our cigarettes, or outright steals them. He contributes the least toward the bills (due to the fact that he makes the least money in the household), but complains that we don't have a bigger place, better things, etc. He'll make a mess, eat off the dishes, leave his trash on the end table next to the couch, but hell will freeze over before Bob will do a dish or help pick up the house. He's constantly bumming money from people, even my uncle, who doesn't make much more than he does! In fact, when we informed him that Sis and BIL had invited him and that he was more than welcome to come with us, the first words out of his mouth were - get ready for it - "Hell yeah, free food!" And I don't think he was saying it in a joking way, knowing his ways of being a moocher. The only reason we haven't kicked him out yet is that he has no family up here and not many friends, and I couldn't just kick someone out on the street like that. Sis knows how he tends to take advantage of Mr. Bottlecaps and myself, and she still invited him to our family gathering. As PPs have said, I think it was a hurtful message to Sis and BIL, basically saying, "I don't want to spend time with you guys." Sis and BIL know that Bob has no family up here, as he's originally from Alabama (he is Mr. Bottlecaps' brother's girlfriend's brother - confusing much? LOL), so they were trying to be nice and include him, and I know it hurt their feelings, even if they didn't say, that he didn't come just because he didn't want to.
3) Bob claims to have severe social anxiety/agoraphobia. I say "claims" because, while I'm not a doctor, he only seems to point it out when it's convenient for him. He has no problems going to the bar on our dime (we actually had to put a rather abrupt stop to that - he was doubling our bar tab every weekend and very rarely contributed toward it, so eventually we had to tell him that if he didn't have the money to pay his own way, then he couldn't go with us). He has no problems going to parties where there will be free beer and food. But at work, he says he can't go to the dining room and help bus tables when it's super busy, because of his anxiety, but still expects us waitresses to tip him out. He couldn't go to our family dinner because he didn't want to be around people he didn't know, but expected food to be brought home to him.
Overall, Bob is a horrible roommate and Sis knows this, as she's listened to me talk about it before because it does cause a great deal of stress for Mr. Bottlecaps and I. We let him come up here with Mr. Bottlecaps (with me paying for his bus ticket) because we wanted to give him a second chance, as most people deserve one, and now it's all worn out. He's made up excuses not to do everything he said he was going to do up here (get his GED, get a decent job, help us out around the house). Sis knows I'm just counting the days till he leaves to go back to Alabama, and despite all that, she still invited him to join us. Although I felt kind of bad telling him (when I saw him trying to dig into Uncle's plates, as Uncle wasn't home from work yet), "Sis sent that food home for Uncle since he wanted to be there but couldn't and still helped me with the casserole," I didn't disagree with her decision in the slightest. If it had been left up to me, I would have brought him home food but resented it later as a PP had said, but it was my sister's house, mostly my sister's food and it was her decision based on past experiences of people not wanting to be there but still expecting food.