I recently got a high school graduation invitation for my niece. I don't want to attend for several reasons. I don't like to fly and they live a couple states away, I don't like my sister's in-laws, and my sister and I have a strained relationship. She attended my daughter's graduation last year and I feel the only reason she did it was to make me feel obligated to go to her daughters. I would like to just send a card/$$ and explain that I am not able to make it. I think my sister would be upset although I know my niece wouldn't care one way or the other. I know this would cause a bigger riff between us. I just don't know what to do.

I think, if your niece will understand then she is the only one that matters here. Just give her a sweet card and if you want put money in it, you don't have to explain the whole situation to her. Good luck. Aren't you so proud of her? Woo Hoo! She did it!

Yea, I think as long as you have the courtesy to give her a card she should be fine. I'm graduating tonight and I'm soooo excited. I can't believe it came so fast.:D -Spidey

No, you do not owe anyone your attendance at a graduation. If you do not want to go, then stay home. But please acknowledge your niece's achievements (I think you would) as her graduation has nothing to do with your sister.

No sense in going and making yourself miserable and perhaps that spilling over to affect others.

Send the card and gift, with a simple "I am so proud of you. Congrats!" or similar. You are under no obligation to explain your non-attendance.

I got an invitation to my cousin's grad. And I guarantee they don't expect me to show up. It is more of an announcement, in my opinion. We will send a gift with a card.
Since this is your sister's child - just explain before hand to your sister. Then send your gift.
It should not be a big deal... hopefully.