My [24f] older sister's [50f] husband [55m] creeps me out and I don't know how to bring it up to her.

Hi hello Dating.mobi,
I'd like to preface this by saying that my sister and I have a wonderful relationship, are very similar in every way, have literally never fought (by virtue of our age gap), and I feel able to talk to her about virtually anything. That said, she and her husband have been married for 14 years and have a 13 year old daughter together, my niece, with whom I'm also very close.
Historically, her husband and I have gotten along pretty well, easily joking together and interacting at family gatherings without issue. This has been true since they started dating when I was very young, and is even true now, as I've started trying to put distance between us whenever possible. We have no discernible animosity, neither in his eyes, nor in the eyes of our family members.
My family lives thousands of miles across the country from me, as has been the case since I left for college at 18. As such, since adulthood, him and I have never spent more than a couple of hours per day, or at most a couple of days (evenings, really) together, at a time. The first time I recalled noticing anything wrong was when my whole family flew out to visit me for graduation; they rented an AirBnB for all of us to stay in for the week. During this time I began to notice her husband frequently staring at me more often than I was comfortable. He'd look me up and down in ways that I didn't like, and even when not directly speaking to him I could feel his eyes on me.
During this week that we spent together, a close friend of mine [24f] mentioned to me that he made her uncomfortable as well, and that she didn't like how frequently he made an effort to insert himself into our conversations, hanging out with us after everyone else had gone to sleep, and the ways in which she too would catch him looking at her. Though I can't remember what specifically, she and I experienced one particular evening in which he made comments about our appearance that left both of us feeling uneasy. This all came to a head when one night, as I spoke to my parents about something, he inserted himself into the convo and, using his hands to explain something, made a distinctly/obviously lewd gesture that he had NO REASON to make, and which he did so while maintaining full eye contact with me. From this point onward, for the remainder of our time together, I did my best to keep a distance from him, and acted less warmly than I otherwise had. When my family flew back home, I remember talking to my mom about how uncomfortable I was by him looking at me, and she remarked that she had observed the same thing and recalled thinking to herself that he had "some nerve" to be doing so.
This was a year and a half ago.
Six months ago, my sister and I started talking about taking a vacation together, our first ever "sibling trip." We only ever spoke of the trip in the context of "us" and so I assumed this trip would be her, myself, and my niece. The week before my sister went to book tickets, she called and asked me whether I would have any problem with her mother attending as well. Her and I are half siblings with different moms, and she was concerned there might be some animosity if only her mother was invited and not mine. I said it shouldn't be a problem, since my mom can't often take off from work for extended periods of time. At the end of the conversation she said, "okay great! It'll be you, myself, [her daughter], [her mother], and [her husband]." She had never mentioned him being invited before and I was surprised that she thought to ask for permission to have her mother attend (someone whom I love and get along with well), but not to invite a man (which as I'm sure you understand, changes the dynamic of an otherwise all-female vacation rather drastically).
At this point, I no longer wanted to go on the trip, and agonized for several days over how to back out without directly saying that her husband looks at me inappropriately. I spoke to several friends and my mother, all of whom suggested I try and do so without disclosing this as the main reason.
I should mention that my sister has spoken to my mother in the past (over the last two years or so) about being at a point where she no longer feels she really wants to be with her husband and might like to leave him. Her reason for not doing so is that her daughter is very much a daddy's girl, and she worries that her daughter would be furious and unforgiving should she initiate a divorce. My sister has never spoken about this to me directly.
Anyway, a few days pass and my sister keeps reaching out for confirmation about our trip. I finally wrack up the courage to say that, as much as I'd love to take a trip together, I had been excited about a "girls trip" and did not want to encroach on what was now more of their "family trip" because her husband was involved. She explained that he had never been to Europe and was looking forward to the trip, and also that she knew he would feel lonely and left out if the whole family went without him. They rarely get to take big trips like this as a group. She asked me, semi-seriously, but also somewhat jokingly, whether I wanted her to uninvite him, which of course I couldn't say yes to without being rude and also needing to provide further justification. Horrified and nervous as all hell, I told her that I instead would love a raincheck for a girls trip that we could do with just the three of us, and she understood. Didn't ask any further questions. A few weeks later my niece sent me a text asking why I had backed out of the trip, prying several times for more reasoning. I'm unsure whether this may have been suggested by my sister. This was maybe three months ago.
Fast forward to today, I'm going home for the holidays soon. My sister has invited me to spend a few days with them at their vacation home (something I've done in the past), and again I'm faced with the same dilemma of wanting to limit my interactions with him as much as possible, such that I'm definitely reluctant to spend multiple nights in close quarters with him.
I have no idea how to get out of speaking to her about this. I know it's a conversation we should have, but it's ideally one I would have liked to have after they potentially separated. I worry about the (additional) irreparable damage this would cause to their relationship, and the possibility that it could even push forward their divorce. I worry about the inevitability, should they not separate, that she confronts him about this and all future interactions I might be forced to have with him after that.
I have no fears that he might ever try to escalate things physically. This is purely emotional discomfort. He makes my skin crawl, and in the last year and a half I've done my darndest to be much more cordial, less warm, less joking, with him.
TL;DR my sister's husband looks at me inappropriately and I don't know how/whether to bring it up with her, or how to keep making excuses for why I don't want to spend extended periods of time with them as a family.
Please, dear people of Dating.mobi. Help me out here. How do I speak to her about this? If I shouldn't, how can I discreetly continue to maintain distance from him? I'm at a loss, and growing increasingly nervous about being forced to talk to my sister about it.


Source.