Should i go to my sister's graduation or go to a field trip?

You go and support your sister by going to her graduation. It's a big moment for her. That's my vote, if that means anything. When I moved out after my high school graduation, I still had three siblings at home. I made sure I made it back home to each of their graduations.

What ever decision you make, you are the one who will have to live with it.

99% of me says you go to your sister's graduation. The other 1% says it depends on the field trip. If it something that is going to help with your future, like something that is going to give you some kind of experience that is going to help you get into college, you should do the field trip. But if the field trip is something that is just going to be "fun", that 1% goes back to your sister's graduation.

Sister's 🎓 only happens once in her life.

Take the Field trip only if its medically necessary ;)

I would have skipped my high school graduation if it had been optional.

I skipped my college graduation and went to Europe. 

(The mailed me my diploma.)

The importance to your sister of your presence at her graduation is probably a relevant factor.

Talk to her and your parents to help you decide.

That all depends on how important your field trip is.  We don't know how old you are, which graduation this is for your sister, or how far away you would have to go for your trip.  If the field trip isn't a big deal, then of course, go to your sister's graduation.  If it's a once in a lifetime opportunity for you to go on the trip .. I'm sure your sister will understand if you don't want to miss out.