The main conflict in the book at the moment is that Phoebe is still getting used to having Leonard around and she seems to have a real problem with it. Not all of it is her fault considering Leonard can't mind his own business and seem to get everyone into a mess they don't want to be in. For example, when they were eating out for Thanksgiving dinner, Phoebe's father comes into the restaurant, when they go to leave they forget their coats and go back to retrieve them, but when they get back to the table Leonard is having a conversation with the man. Phoebe's mom really wasn't happy with that and threatened that if he ever did that again she would kick him out. Phoebe was shocked and her sister was pretty upset to.
This conflict could be resolved by them sitting down with Leonard and telling him exactly what they feel when he does something like that. They could explain that it's o.k. to get involved in some things but when it comes to personal family problems he needs to stay out of it unless it involves him directly. They need to express whats going on so he can understand what it is he's doing wrong instead of sitting there clueless and having no clue as to what he did wrong. He doesn't seem to grasp that it is wrong which is why this conversation is necessary.
I would deal with this conversation exactly how i explained in the previous paragraph. I would sit down with him and have a conversation about the things that are o.k. to do and whats not o.k. with me. I would want him to understand rather than wonder. I would want him to know what upsets me and what i expect from him.