is it wrong to like/have a crush on your good friends sister?

It's not technically 'wrong' to have feelings for someone else regardless of your relation to them. However if you pursue it there are obviously going to be some complications.
Firstly of all, regardless of whether or not she's your good friend's sister, you need to consider the age gap. I have no idea how old she is or how old you are, but if you're in your teens I wouldn't recommend getting into anything with anyone where the age gap is 2 years+.
Secondly, you need to figure out how much you like her. There's no real point in making a big deal out of this if you think your friend's sister is attractive (as opposed to being attracted TO them. There is a difference.)
Thirdly, if you do decide to pursue this, you need to think about how this'll impact on your relationship with your friend. As most guys are generally really, really protective of their sisters you might want to take the pace of your relationship with his sister quietly and slowly.
anyway, it is ultimately up to you to decide whether or not it is irresponsible or wrong, but sometimes you really can't help falling for someone even if its against your greater will.

How do you get somebody to stop liking you? I want to be friends with him, but I can't handle any of this. This being the feeling of paranoia I get when I talk to him, out of fear that he'll misinterpret friendliness for romantic interest.

Well truth of the matter is, and I know you're not going to like this answer, but you personally, can not do anything to get some to stop liking you (unless you really want to do something societally or morally frowned upon, but then you'll have to live with the consequences of your actions which is a jillion times worse to deal with than unrequited romantic feelings.)

However, if you do want to be friends make sure he knows and understands CLEARLY that you don't want to be in a relationship now or the new future.

Also, you don't be to pushy in regaining the previous friendship (he'll probably misinterpret your friendliness) but try to not isolate or ignore him (then he'll probably feel like you're trying to eradicate his presence in your life.)
I know finding that balance isn't easy especially when things start easing up between the two of you, but keep your head focused and just keep an eye out on small characteristics that hint at him trying liking you again. If he does, then you need to lessen the amount of friendliness you show him.

I'm so sorry this is so long; I have far too many things to say for such a small text box ahah.

At what point do you think a relationship, or friendship, becomes unhealthy?

Well, it always depends on the situation. But most definitely when there is a clear power inbalance, like when one person becomes too dominative and manipulative and the other person becomes too passive and submissive you've got a problem.