Next week is my sister's graduation and I promised myself at the beginning of my transition that I would be full time by then. However my parents are still don't agree with me taking hormones and being trans all in itself when I told them 7 months ago. A few of my relatives and family friends are gonna be there along with my grandparents. They all don't know but all of my siblings and immediate family does. I asked my sister if I could and she said it would be okay to her and that she didn't care. My parents however are the ones who are quite nervous and still act like me being trans is shameful. Part of it is cause my grandmother(who I stay with) and most of my parents friends are religious and they're scared that it would reflect bad on them. I keep telling them I'm gonna have to come out eventually.  I'm getting to the point of blackmailing them and just coming out on Facebook but idk yet. I need advice should I go and out myself to everyone (with my sister's permission) or should I remain in the closet? I can't live like this forever.

This is only my opinion, but I would let your sister have her day in the sun drama free. 

That's kinda what I was thinking too.  There will be other opportunities to come out which don't coincide with celebrating someone else's achievements.

Think of it this way: when you come out, you'd prefer everyone to be focusing on you, right?  If you come out on your sister's big day, they'll be focusing on her instead.

What's your reasoning behind wanting to come out to those more distant relatives?  You mentioned you're already out to your parents and close family.  Is there a specific reason you need to come out to everyone else on the same day?

Good on you for checking with your sister first. Even though she said yes I'm still inclined to agree with the others above. It's her big day, probably more stressful than she realises, let her have it drama free. Even though she said yes if you outing yourself goes badly everyone else will see you as the one who "ruined everything", that's a lot to recover from.

I 100% agree with Ms Grace. Even though you wouldn't ruin the day or moment by coming out some family members would see it that way and justify how they treat you based off of that event.

I would plan an event where the most important members of your family attend that you create then tell them in that fashion. That way the event is yours and yours alone so nobody can ever say that you ruined so and so's "perfect" day.