Even though Pip is still a Piplup, he is the oldest of seven kids by his Emopleoon mother and Blastoise Father. Even though his parents don't agree with his choice to not evolve, they understand that it is his choice to do as he wishes, and still treat him like the eldest child. Memphis, the Empoleon beside Piplup, however, is the youngest, the 'baby' of the seven kids, and does not like that his Piplup older brother gets treated more like a fully evolved Pokemon that he does, when he is fully evolved and Pip hasn't evolved once.

Pip and Memphis © me
Empoleon and Piplup © Nintendo