Question: When Uncle Buck is going to the party to get Bug, he is stopped at a red light and there are two parents in the car next to him. The father says, "don't go in there with that hat on, they'll kill you". Why would someone be killed for wearing a hat?
Answer: The Uncle Buck movie was made in the late 80s, near the end of the Cold War. And Because the hat resembles the Russian ushanka worn by Russian soldiers in that time, it can be said that wearing it might anger some, especially young Americans opposed to world events at the time.
Question: In the scene where Buck's brother rings him up to babysit there is a body in Buck's bed. In the scene before Buck is at a bar but brings no one home from the bar, so where did the the extra person come from?
Answer: You can't actually see a person, all you see is a human-sized lump under the covers. It's possible that it's just bunched-up blankets, some pillows, or even clothes.
So, I just noticed this in the movie. If you go at about 10 minutes and 21 seconds, you'll see the blankets move behind them real fast while he's on the phone.
Question: The older sister seems to hate Uncle Buck. Why does she hate him?
Answer: She doesn't seem to really hate him, per se. She's a rebellious teenager going through a hard time in her life, pretty much giving all adults/authority figures a hard time (including her parents). Like a lot of teenagers, she doesn't like being told what to do and thinks herself far more mature than she is. She starts off on the wrong foot with Buck before he even shows up, since she resents her parents for having him come in the first place; she thinks she's old enough to a) be left alone, and b) look after her two younger siblings. So, when he shows up, she immediately defies him and refuses to acknowledge that he is "in charge", since she is, in her mind, not a child anymore. Her relationship with him eventually thaws as she realises that he genuinely cares about her (something she doesn't think her parents do).
Answer: It's never explained, but the best guess is the hat was an Ushanka hat which is often associated with Russia. At the time, there was a lot of Soviet Union controversy so being associated with supporting the Soviet Union could have angered many people.
Chosen answer: He's being sarcastic, as Buck says in the beginning some of his hats anger a lot of people, which probably connects to when his hat gets taken by one of the teenagers at the party.
Buck is actually referring to his aviator style hat in the earlier scene about people being angered by it. He's wearing the fedora style hat in the scene en route to the party. But I agree the guy was being sarcastic about teenagers.
The black purse bedside to the possible 'pile of bunched-up blankets' looks like Chanice's from later scenes; suggesting the writers or Hughes changed the script after the bedroom scene was shot to Buck's simpler but delightfully funny half-conversation with her on the phone.