Answer: He is more comfortable with this dog than with his distant relatives.
Explanation: The other options don't necessairly make sense, at least with the exceprt you provided - I wish you had shown the entire passage :(
However, this can be seen with, "Buddy is the perfect houseguest" and similarly with, "But Aunt Clara claims to be deathly allergic to dogs. She swears she developed this since the last reunion."
This further indicates tones of sarcasm when Kevin speaks of Aunt Clara.
With Buddy, on the other hand, and about Buddy, Kevin only speaks admirably.