The correct answer is Mi hermana va conmigo
1. estamos
2. estoy
3. estan
Ellos - it's the male version of 'them' because there are 2 males. it doesn't matter how many female is in the group, if there is at least one male, it's masculine. (like, 99 female and 1 male would still use the 'ellos' despite the fact that the majority are female)