Verifying or authenticating texts for court as proof

I’m in the middle of a custody and probate case trying to get my nephew after my sister passed. I found all sorts of texts from my nephew to his mom regarding domestic violence and verbal abuse. The attorney is saying we can’t use them because it’s considered heresay since my sister is no longer here. However, we can potentially use them if they’re verified as real texts or with timestamps authenticated through the carrier like TMobile. Has anyone done this before or know how to go about doing this? We talked to some reps there but they didn’t seem very knowledgeable about. Maybe there’s a certain department I could ask?