Coming out to first family member (Non-binary Trans*)

So I guess this will be my first post in these forums, so here goes...

To start, I have been questioning my gender for nearly a year (This upcoming August), and I have been slowly becoming more confident in my gender identity, and I feel almost ready to come out to some of my family now.
I know that it is suggested to have some supports for just in case it goes wrong when you come out to parents, but in the past year, I haven't had much, if not at all, any possible support due to lack of close friends (I've always been a loner/outsider). I have told a few friends early in my questioning (Which I sort of regret), and they supported me, but since then I have lost many of those friends from unrelated issues.

So now, when I find myself ready to come out, I'm left with this predicament of not knowing what I'd do if something went wrong (Even though I know the risk is low, I am a paranoid type of person).

Now I am tempted to tell my sister, whom I have known to be a LGBT supporter for a long time and recently I found out she is pansexual. My issue is that, being her "little sister", she often does not take me seriously, more so now because I can sometimes act seemingly "immature" because I tend to talk a lot more because I am lonely and tend to get a bit giddy when I get to talk to anyone relatively close to my age group. I want to tell her because if she does take me seriously, I can gain a lot of support from her.
I am also not sure whether to tell her that I am still questioning (I'm pretty sure now), or somehow describe to her what my weird non-binary gender is (Demigender, more specifically, Demimale).
Along with this, if I do decide to tell her, it will have to be tomorrow. This is because she leaves to go home in a different province then, and I will not be seeing her for a long time in person, much longer than I plan to come out to our mom (I plan to come out to my mom, and then everyone else, early September (Before college starts), and I probably won't see her again until Christmas).

I feel like I need some sort of guidance here, I feel like I need opinions on this issue, even though I think I might possibly already know the answer to this (Yes, I am silly, but I guess I am just scared).

:help:​

>I might leave the actual coming out questions for September in a different thread later<

 

Thank you so much for the opinion, though I'm starting to feel nervous.
Right last night before I fell asleep that it crossed my mind that it would give her time to think, do you think a quick private chat with a promised more in-depth explanation later would do? I have an extremely small window in which to do this today, so I'm crossing my fingers here.
By demimale, I mean that I identify as mostly agender, with some feelings of being trans*male. I'd say the ratio of the two would be 70/30. It's not a very well known non-binary identity as far as I know.