You're in a bit of a mess here, but I'll try my best to help.
(Also, gosh you're popular with the boys!)

1. I can relate. This is something you need to sort out, because it's disrupting the rest of your life.
You both like each other but don't feel like you're ready for a relationship yet? You need to set a boundary. It sounds dumb, but you need to choose which one you want and try to stick with it. If it's not working out, don't press it. Take a break from each other if you need to.

2. First off, this guy really needs to step the fuck off. Be firm with him, because that shit's not appropriate. Try to talk to him about it, explain that you see him as nothing more than a friend and you don't want him to act that way around you. If you really, REALLY have to, you can try avoiding him, though I wouldn't recommend it.

3. God, you're one damn responsible person. Taking on something like that is really impressive and it's normal that you're having a bit of difficulty with it.
Maybe you could try speaking to your sister's teachers about it? Or look into hiring a tutor of some sort, if you've got that kind of cash. Try to her to take initiative in her own learning by making it interesting to her - then she won't need as much help from you.
As for your parents, have you explained all the stuff that's going on in your life at the moment? I know how hard it is to have pressuring parents, and you HAVE to explain to them. Sometimes you just need to have a heart-to-heart conversation to clear these things up.

4. I'm in the exact same situation. Try to stay relaxed about it, but get the work done. Prioritize your life - what things are most important, and how can you get them done? Put everything in order and stuff like studying is easy.
Don't let it intimidate you. You CAN do it, you just need to get yourself in the right mindset.

5. I understand you don't want to hurt him, but you need to let him know you're not interested. Same thing with 2, I guess. Have a chat and get it across that you'd really prefer not to.

I'm not helpful at all, sorry. But I hope everything goes well for you - I'm sure it'll all work out.