Ya know, I'm the kind of person that would be like "hey y'all, I'm part of the family too so I should be in this picture" Or I'd just jump in the pic anyway. Maybe it has gone on so long without you saying anything that they all just assume you don't care to be in the photos. It's time to say something to your sister and also re-address it with your mom. Tell her you aren't being childish and that it really does hurt your feelings. I'm not a confrontational person but I have learned that no one is going to stick up for me but ME. Don't let them hurt you anymore!
Originality is the art of concealing your source. ~Franklin P. Jones
There is a very fine line between "hobby" and "mental illness." ~Dave Barry