Did anyone catch last night what Teresa said to Joe when they were talking at the pool? She said that when she first married Juicy Joe, that she still put then-single Joe (her brother) FIRST before her husband.

It sounded like she was expecting Joe to do the same for her and put her first before his own wife. Since that isn't happening, Teresa is upset at Joe for not being a good brother. Of course, she's mad at Melissa too for 'corrupting' her brother. Imagine that!?! How dare Joe put his wife first!?!
If that's really and truly where Teresa is coming from, this sure seems like the root of their problems. None of this has anything to do with what was written in that cookbook, who copied who, who's a better aunt than who, who bought what present for which kid, etc, etc, etc. Those are all just symptoms of the real problem.
ETA: That would explain too why Juicy Joe hates Joe so much. I'd be pretty resentful too if my spouse put their relationship with a sibling ahead of our marriage.
These people really do need counseling.