A. Rules are different from house to house, and kids know it. Most parents know this, too -- after all, it would be presumptuous to tell other moms how to run their homes or what rules to follow (unless we're talking safety violations or, say, feeding hamburgers to little vegetarians).
But that's not to say you should plan a morning-TV marathon during the visit.
My girls' friend Sydney isn't allowed to watch much TV at her house (I know this because it's come up in conversation, not because her mom demanded she not watch when she's at our place), so when Sydney comes over, I'm less likely to agree automatically to requests for SpongeBob and more likely to point out fun things they could play instead of sacking out in front of the box.
So when your nephew is over, don't think about what your kids can't do, but consider all the wonderful stuff they can do together. At the same time, don't hide the fact that your kids watch. Tell his mom up front: "I know you don't really like Mikey watching TV, and I've got a bunch of activities planned. But there may be a time when they take a break and enjoy a quick show -- I hope you don't mind." If she does, she won't send him over. But something tells me she'll be okay with it.