toxic mom

long post warning...

my grandpa just recently passed and i had to travel with my family to see him. it hit everyone pretty hard... but my sister is displaying it the most. nothing against it, she grieves however she needs to. but anytime my family gets together, my mom is extremely aggressive and mean to my sister. i hate getting the family together because she always does this and think she's always right... is anyone else's mom like this? my sister is older so she's thinking about just cutting ties and i hate being in the middle of all this... please tell me someone else has dealt with this :(