I need help this is not a joke. (NO RELIGION PLEASE)

Try asking her nonchalantly, don’t go to her and tell her you have an obsession about her feet. I think that would be creepy for her no matter how you tell her.
Just tell her that you have a great appreciation for her body , and that you would love to take a few pictures of her (example “You know (name) you’re really pretty, would you mind letting me take a few pictures of you?”). This would really help if you are very much into photography and already take lots of pictures (You can always make an excuse up like “It’s for practice” or something). If she agrees, then start by taking a few pictures of her normally, keeping the air light and maybe even a bit on the humorous side.

Eventually when both of you relax and start having fun and enjoying the photo shoot causally ask her to pose in a way that expose her feet, or, Depending on your relationship with her come out and ask her if you can take some photos of them, keeping it causal and in a joking manner.

In this way you might not even have to tell her about your secret obsession, just remember to discreetly keep the photos hidden.

If you really want to tell her though, build up your friendship with her before doing so, just coming out and saying that you are attracted to her and her feet would do nothing but scare her, unless she’s into that kind of thing. Usually though those cases are rare.
And in my opinion I see nothing wrong here , since you’re only related to her by name (I believe?). But no matter how much you’re attracted to her remember to respect her current relationship, trying to force yourself between her and her bf will only turn your situation down a road you probably don’t want to take.

Ah well, in that case all I can offer is to work on befriending her in a non romantic kind of way and then sitting her down and talking about it, if she’s a nice and understanding person and you don’t come off too creepy about it all she may well understand. It’s hard to give advice in a situation when I don’t know what the other person is like personally.

Best I can say is to just trust her in not talking about it if she refuses. And truthfully, why are you worried about what others think of your obsession? Feet fetishes are actually quite common I’ve even seen Japanese tv shows about it. And if your family/friends think you’re weird for it then who cares? Fetishes, from what I know of them, are almost as bad as OCD. They tend to consume and fixate the person they are attached to, to the point where they can’t control them and makes the person feel like a freak/creep (which usually leads them down a path of sorrow when there’s no need for that).

The same goes for your -more than sibling affection- for her, if those around you find it creepy just tell them to fuck off; if you were blood relatives it would be different, but you’re not, and sure some people may look down on it, but think of it this way: you can’t help it that your parent and her parent hooked up and you found yourself attracted to her.

If anything you could always disown your family. Which would work well if they come to find out about all of this and are cruel enough to turn away from you.

As for your loneliness, what exactly do you mean by it? Are you lonely in the sense you need/want partnership, or friendship, or you feel alone because you believe no one would understand you, or something like that?

Hm, I read your other posts just now. I would say I’m sorry, but that would be a form of pity and you said you would rather not have that.

I’m afraid I’ve never been interested in love and relationships, in truth I don’t even see why most are consumed by the need for either of those things, so there’s not much I can offer you.

What little I can though, in this day and age, would be useless to you. There are few women, or men for that matter, today that can survive in a relationship based solely on intimacy without sex however you seem like you would be rather good at making up for, as you say, your inadequacies (The idea of a man that would never cheat on his love is shocking to me).

The last bit of advice I can give you, as generic as it is, is to talk to a doctor, if it’s uncomfortable for you to talk to someone you know, talk to someone you don’t. Ask him about ways to decrease your libido, to make it less painful for you.
There is also the option of Castration, which I know that most men find this idea appalling but in your state you don’t seem to really have anything to loose. But I hear it is a great way of controlling a man’s sexual desire.

Another way is to (at least try) to look beyond your desire for companionship for something else that interests you; Maybe take a trip around the world?
Eh that still sounds like generic advice, but I’ve personally done it (not traveling the world mostly to the beach for a weekend) and from it I’ve learned that by getting out of the situation that’s causing you pain like what you’re feeling, helps to give you insight that there is, indeed, more to life then what first appears.
Besides, you never know, you may find a hot French babe that likes guys like yourself?

In conclusion,

There are a billion or so people on this earth my friend, and I really don’t believe that there is no one out there that wouldn’t find you at least somewhat attractive. Shun this idea all you like but take it from me, a woman really in no better shape than you (take your abnormal genitalia and replace it with an abnormally small breast); If there are men that could be attracted to me, there has to be a few ladies out there for you.