You are really reading too much into too little, dude. In what universe did I say I was set on marriage?
Ok, reading back through I was mistaken. My advice is still valid though. To get back on track though:
I'm laying a breadcrumb trail; she can choose whether to follow it or not. No deception involved, beyond what anyone else does on a regular basis (unless you're telling me that you've never done anything similar with your crushes? Always totally upfront from the start?).
The first sentence sounds creepy and manipulative, which is where I was coming from before. Yes I often spend time hanging out with someone before I ask them out. Because I'm generally not a weirdo who decides someone is dateable based solely on looks. There's nothing wrong with casually hanging out with someone or chatting someone up online to see if there's any compatibility there. This is the essence of how tinder works, after all. But calling it "a platonic date" puts this whole creepy vibe on the thing, and frames the whole thing in this really manipulative light that makes me feel gross. Here's where you're going wrong, you're hedging. You're saying: "I'll present myself as a friend, and if I'm lucky maybe it'll turn into surprise sex (in a non-rape kind of way), if not, I'm still her friend. But here's the thing, dude. Friendship is not and should not be a consolation prize. Nor is it a waiting room that you chill in until you can graduate to the next level. If you are only being nice to someone because you're expecting something out of them at some point, newsflash: that's not friendship. This isn't to say relationships can't develop out of friendships. In fact they often do. But that's not what this is (or would be).
This is the sort of thing I'm talking about, right here:
I'm literally just taking her on a platonic date. If we do end up having sex, it'll be just as much her intention as mine. I don't even think it's likely.
So which is it? A platonic date, or a romantic date that might turn into sex. Because you can't frame it as a platonic date and then act as if sex is on the table. That's seriously uncool dude. This is the friendzone fallacy. If I present myself in a friendly light, then maybe she'll realize I'm a catch and want to bone me at some point. This doesn't work, because if you come in presenting yourself as a platonic friend, then she is going to see you as a platonic friend, and not as someone to possibly bone. Then you're in the friendzone. She's not going to make the move, because you aren't seen as a possible romantic partner. And if you make a move, she'll probably go: nah. And if you wait forever and then eventually work up the courage to make some grand romantic gesture (which is how this normally plays out) then the instant you do it becomes immediately apparent that friendship was never the end-goal and you've just outed yourself as a horsehockey, manipulative person.
So to summarize: don't do this.
Do:
1) Casually hang out with her: invite her to parties, watch horsehocky on skype together, maintain a casually flirty text correspondence
2) If you like what you see, and if you're getting good vibes from it, ask her on a date. Say "hey I like you, and I feel like you like me too, can I take you out to [dinner, a movie, a museum, whatever] sometime?
3) Don't do 1 for more than like 2-3 weeks
tops before moving on to 2. horsehocky or get off the pot.
4) Since she's your cousin (even if not by blood), she might find it fairly squicky. If you're getting good vibes from your flirtations, then you're probably going to want to broach the subject about if she's even into the idea of you two being an item before you ask her out. Because not everybody (in fact most people) sees this in the light as you. It's possible she sees you as a strictly family, and so places you automatically in the extremely non-romantic friend area. Or not. You'll never know unless you ask.
5) Or, you know, you could save yourself the trouble. Long Distance Relationships are hard enough as it is. Let alone with someone who might think your being a cousin is 100% a dealbreaker. Tinder, coffee shops, bars, parties, and social groups are great ways to meet people who are more open to the idea of boning you.
We don't live in the same town. I'd have to call or contact her online and make my pitch. Maybe the movies could work, but I've never understood why people use them for dates... I'm always too focused on the movie.
Clearly you're not doing the movie date right. Then again, I don't think they're particularly great first date ideas either. The whole point of a first date is to get to know the other person and see if there's compatibility. So let's go to a dark room and not talk, look at each other, or interact in any way whatsoever.