No, I don't think you're interfering. You're the mom and you don't have to coparent with your X's latest fling. I say this as mom, but also as someone who was a stepmom for a long time. As much as I loved my DSS, I never dealt directly with his mother. I left that to my X. We kept our distances, were polite to each other and that's it. I didn't want her to feel like I was sticking my nose where it plainly didn't below, and I only stepped in when she specifically asked me to.