Would I Get in Trouble if.?

Okay so my gym teacher was at my brother's (who is a freshman in high school so she obviously taught him) volleyball game. My sister was there and she sent me a pic of her and I want to post it on Instagram but would I get in trouble for that? The picture is fine; its just of her sitting and its not offensive or anything. I'm just not sure if I should. So is it okay or should I just keep the picture to myself?

I doubt it would be a problem but it makes sense to ask her for permission first.