What is the power of the seventh Witch? What exactly is her power? 

Why did Miyamura's sister, Yamada, and the president not get affected by her memory erase? 

I understand that perhaps she couldn't erase Yamada's memories, but then how did the other evade the memory erase?
The 7th witches power is the the power to forget everything about the witches. 

Miyamura's Sister ran away from her and escaped so was not affected. She's hasn't returned to the school since.

Yamada's power reversed the power and everyone else forgot about him and the witches.

The president was not affected because the student council president is the only student who may be immune to the witches' powers.

There's some fan speculation that the presidents are people who have Yamada's power, or have had.