1) I think a tupperware party for a Kitchen Tea is fine, so long as your sister doesn't expect gifts on top of that. If I understand it, your sister will get some of the cash generated from any sales made at the party? That should be "the gift". If she insists on wanting gifts on top of whatever sales are made, I'd distance myself from this party, and refuse to have anything to do with hosting it. Oh, and as PPs have stated, it should be made clear upfront to guests that it is in fact a tupperware party.
2) I'd encourage you to try your best to be there, getting ready with your sister on the morning of her wedding. I like the idea about staying overnight at your parents' place. Or perhaps you can take a taxi, or public transport, to their house first thing in the morning?
3) Again, it's not rude to just attend the dinner part of the Hen's Night. Slightly off topic, but I personally don't see what's so outlandish about going to a nightclub if you happen to be a mother. No one's saying that you HAVE to dance with strange men. You can just have a drink (soft drink, if you like), dance a while with the girls, then leave.
4) This is tacky, and as a sister, I personally think it's ok to tell her that she shouldn't EXPECT her in-laws to fork out once she's married.