who's right in this situation, me or my sister?

[Ekaterina]

neither was right.
if your sister did it before then shes being a hypocrite but probably feels bad for your mom and was trying to "stand up" for her.
and it was wrong of you to write that about about your mom; even though the comment wasn't too harsh. do actually try to see how you would feel if your mom had twitter and posted up "i cannot stand my son holy ****"
its better not to trash people in public. most of the time, it makes you look bad.
im sorry about her blowing up the 66% situation. parents can be buttholes. sometimes because they care.
but i wouldn't let it bother you. as long you know you tried. we all get bad test grades every now and then.