It's like taking care of things in their order. It's nice that the niece thing may have provided an opening.
The separation process is emotionally chaotic. Communication skills that were weak usually don't get better.
I guess it's like jumping from the smoke to the fire. Take care of the smoke first.. the niece. She's doing yeoman service trying to maintain contact with everyone. Your wife's text was in reaction to the conflict the niece felt.
The calmer your niece is, the manner in which you allay her concerns will directly impact how she communicates with your wife. And you. And the kids. Actions.
Your wife is the fire. She can go any direction. Know yours. Any text to her should take ten words or less.
Otherwise you're reacting to your wife. Not addressing concerns. And in emotional triangles, who ever tries to save the other gets bitten in the ass.
Straight lines of communication. Actions speak louder than words.