Is the niece an adult? If it's been an obsession for 15 years, I imagine she has to be close to an adult at least, or an adult.
Why haven't you told the brother? If he's mean to the niece, is anyone looking out for her? Is her safety at risk?
I don't think anyone can diagnose him, but the most important thing is that someone has to know so she is protected. If she doesn't know, she should. She shouldn't be alone with him.
You say you can "see lust in his eyes," and presumably are certain you are not imagining things nor interpreting things incorrectly. But what if you were to be imagining things, and told the brother, and caused a big rift?
One way to try to be sure is to ask your niece how she feels about her uncle. If this niece has been the object of his sexual obsession for 15 years, unless you are saying he began this when she was a little child, she is certainly an adult by now and you could have this conversation. Can you say to her that you are puzzled by the way he acts around her and ask her if she is comfortable with it? Maybe her response will give you an idea if there is any chance that you are not guessing right.
So she's 31. She is a fully grown, adult woman - a mother, if I'm reading this correctly.
I guess my next question is why isn't anyone else seeing this? Why isn't anyone else saying anything to this woman?
Why would you ask a "leading question" of the niece? Why not ask the uncle?
You are reading a LOT into this. You seem to think that traveling with family is somehow inappropriate or suggestive of something lurid. Are you on these trips? Are you the uncle's significant other? How else would you know about the "sneaky, lustful looks" on the trips?
All you can do is control your part in this - whatever that is. It's bothering you, and has been for 15 years. You can choose to say something to her father, to her, or to the uncle. Either that, or let it go.
I guess it's clear from the kind of answers that you're getting that it's hard to tell what is really going on from what you are describing. You say someone has been giving sneaky lustful glances at his niece for 15 years, it has never escalated to him saying or doing anything overtly improper, it has never been detected except by you, and he has never stopped doing it? Part of the reason this is hard to make sense of is that people with unrequited attraction finally get bored and go find someone else: it's hard to imagine someone staying at a fever pitch for 15 years. The only way I can think of to be certain that the situation is as you have described is if you yourself are the uncle, and are writing trying to figure out or self-condemn your attraction. (It doesn't quite add up that way either, but if you are, keep doing what you are doing, which is to keep the attraction to yourself and don't try to act on it.) Anyway, it would be helpful in understanding this if you would say why you think it's suggestive that the two daughters travel with their father, who you are in this mix, and why (if the uncle is so obvious) they would still want to be around him.