Sisters hardly net you max points on offence and are sure to steal points even if beaten in one go on defence.
So sisters certainly are throughout all devisions a good idea for defence.
Its similar with GG sep droids.
I assume at lower gp and devisions you see more bossk leads at higher gp it's all jango lead hunters.
Nest is awesome in lower gp gac for offence to solo teams, at higher gp you can forget about it, because there wont be a team she can solo and you rather beaf up a defence team with her.
Basically you want to be able to clear fully and put teams that can get holds or steal points.
At 5-6mil gp this concludes to the following best defence teams:
Drevan, gg, gas, sisters, nute + wat, jango and than the ice gets thinner because you need to keep enough for attacking. So the last 2 squads mostly are qira and any 8th squad.