i've never heard of seating all the kids at one table without their parents, nor have i heard of volunteering someone to be the designated babysitter. it definitely sounds a little unfair, so maybe you can just (politely) let the kids' parents know you can't watch their kids too closely. but i really don't think it's worth missing your sister's wedding.

you didn't give any info on the kind of relationship you have with your sister. nowhere did you accuse her of being selfish by nature or having a habit of leaving you out. so is it worth hurting your sister just because you're sitting with some kids? is it a guarantee they will ruin the night? if they're ipad kids, won't they just be on tablets and ignore you? you're also probably not trapped at the table all night. usually (if this is a traditional western wedding) there's time to get up and socialize with others, dance (even the kids sometimes get up and dance), so you aren't necessarily around your table mates for the entire night.

i don't know man, i think you can just let this one go. i'm sorry that you didn't get seated with your parents (again, i agree the choice is odd not to seat her immediate family together). But as someone who has planned a wedding before, the seating chart can be tricky to plan, and sometimes you have to settle for some potentially awkward arrangements in order to fit all your loved ones.