At least people have stopped misunderstanding Roxanne as being malicious or abusing Sherry.
She's a canid, the order of 'eating' is very important. Not to mention she's the senior slave, meaning her position is basically all she has, and is her identity.
Focusing on Sherry first is the same as stabbing Roxanne in the gut and saying "I only care about new things, you're old now."
Even from the perspective of being in the lifestyle, that's a really bad move that young masters/mistresses can make sometimes, and advice I often have to give out to newcomers to the lifestyle who are sent to me for understanding. There's an art to maintaining the feelings of everyone involved in a multi-partner relationship, whether it's purely romantic or collars and leashes happen to be involved.