Summoning ritual will be a success if you've got the correct branch = tree/Dark Forest. But you should throw away the fake branch first before speaking to her.
BTW, you learn banish from Corinna, not summoning. If you wish to learn this skill, just choose it - which class are you playing?
You just have to move to the mage path in the skill/abilities book (click on < > ) and then choose/confirm the spell.
Kiya
