I'm starting to want a pleasing romantic / sexual relationship with my younger sister. Problem is she has a boyfriend. How do I go about this?

@Firedragonco2 that's good advice, but I feel like she's the type to react squeemish about it even though I feel like theres a possibility that we both know we want each other

@samanthaX I messaged her the other night and I told her that I love seeing her happy, I love talking with her and how much we can laugh together, that i feel like we've grown so much closer, and that it really means the world to me and so does seeing her smile, and i left a heart. I also said in the message that I didn't mean to be awkward and that i just want her to know how much I appreciate her.She agreed that we've bonded a lot more recently and we just had casual conversation after that.

Also yesterday she was being excited about going to college and said that she has a hard time hiding her emotions, and i told her that it's really cute.

But everytime I try to leave very small"hints" concerning her boyfriend and advise her on how a real partner should act, she always seems to give him the benefit of the doubt

@Sistersbestlove I took another big step today, so here's an update for everybody:

Me and her were messaging each other about her boyfriend and I brought up how something he did was a bit sketchy. She still was giving him the benefit of the doubt and saying that it's understandable cause he has mental issues. I said I wouldn't use that as an excuse if i were him, and she was saying he finds it hard to talk about stuff and to people. And that it's also different because they don't know each other in real life. I told her that's what has me concerned tbh. She said she would talk to him (about the issues if they become a problem again), if she needs to, and that he's pretty understanding when she brings up issues.
So then I dropped a big contextual hint because I'm fairly confident at this point that she at least has a crush on me.
I said
that I get a feeling from him that things aren't going to turn out the greatest to say the least, and that I care enough about her that I don't wanna see her fall in love with someone who isn't good for her when she could be much more in love with someone else. Someone that deeply cares for her.

She didn't really reply to what I was saying in the message, but then afterward she needed me to help her get something, and when I opened the door it seemed like when she saw me she started stumbling with her words for a second, we laughed about it, and when I came out there she
was surprisingly bright and smiling with me. I'm thinking what I said might have made her a bit extra happy :]

Please wish me luck and send your good thoughts towards our relationship becoming even more intimate 🙏💕
We'll be watching a movie together on the night of her birthday coming up soon

(An update and would like some more advice):
So basically she doesn't seem to be very bothered by her and her "bf" breaking up, and me and her have still been playfully bantering and innocently teasing one another so much that it just feels natural now. Idk if I posted about this already but she has let me caress her arms before when she's shown me her double jointedness of her arms. I've been wanting to get her more comfortable with me upping the touching when we have the proper time alone together. But Idk how really. Because dor one, she's been shy about me giving her hugs before because she's not used to it. Also a good amount of times before, I've poked her in the belly which usually leads to her running and me playfully chasing her and tickling her.
So one time I was trying to get something from her, she was already laying back on my bed, and I was laying in front of her, so when it happened I was able to wrap my arms around her while wrestling to get it back. I know this sounds like a big step in the right direction, but when it subsided and we were still laying close and opposite of each other, I gently stroked her hip and side while we were talking, and after like 5 to ten seconds she said "what are you doing" so I backed off from it because I felt like it was more than she was comfortable with. I've also really been wanting to cuddle with her, and someone said I should ask her, especially if my mental health issues are bothering me, but what I just mentioned has kept me hesitant.
We watched a movie last night where I could of had that chance, if it weren't for the fact that when we were about to lay down to watch it, she said "are you going to give me space this time" because "I like to lean into her". I gave her more space, but as the movie kept going we somehow still ended up leaning closer towards each other more than we started out. At a romantic part of the movie I saw my time to ask her if she had enough space as kind of a hint. She said "I'm fiiinnne". But she said it a certain way, like kind of an almost annoyed tone yet not really, but also in a way where I kind of felt like she didn't want to actually answer what she may have felt and actually wanted me to get closer to her. Idk, it's really hard to say for sure, but the apparent possible boundary cross from before had kept me hesitant from going any farther.

Also, if circumstances go well then me and her could end up moving in together into our family member's house who went to jail. This would be the first time me and her would be living on our own away from family and having pure alone time with no one around. The thing is idk when that will be, could be a few months from now to over a year. I would like to make a new breakthrough by making some moves before then that will get her more comfortable with touch and a bit more intimate. But idk what to do exactly.
Also, my brother who knows has been saying a couple times recently that I need to tell her my feelings soon. I planned to at some point for sure, but idk if I should do it very soon, or try to let it naturally grow between us under the surface more before doing it. I feel like him telling me more than once is a sign that I need to soon, but I'm not quite sure.
Either way, I really want to up the physical touch and make a breakthrough somehow despite me and her staying with family currently. This is what's making things difficult since we don't hang out in her room much. I also really want her to to cuddle with me when my mental health flares up, but idk how to ask her since she's usually out in the family room when it happens. This is a really tough situation