He says that he'll raise her in the Court, and in the meantime become Regent of Temeria, and that once she's of age, she can claim her throne.
Of course, that may not be his true intention. Geralt and Roche comment on the fact that he may have other plans. They also suggest that if Adda doesn't have children, he'll get rid of her and still marry Anais. Otherwise, they expect him to marry Anais off to one of his nobles.
From a practical viewpoint, the ending is the same as if Adda is dead, just the dialogue changes.
Makes sense. I'll still give Anais to Radovid though. I think it's better for both Anais (she will be with her sister) and for the Northen Kingdoms in general. Then I'll finally boot up The Witcher 3 and play that game with my new save import (I lost all my TW2 saves so I had to play TW2 again first before playing TW3). Awwwww yissssss.