I will be in the minority here.
First off, I don't think anyone could say that you OVER reacted. All you did was calmly ask your mom not to do it that way next time because you were uncomfortable with it.
I think at their ages (esp the boys) most kids would want their privacy. It's hard to teach kids that their body is their own and private if they are asked to shower with "strangers". If they were kids that spent a lot of time together, I doubt it would bother me much, but given that you said they are kids that see each other maybe once a year? I would do exactly what you did.
I'm sure it didn't scar the kids, but I'm with you as well that 10 is pretty border line to be in the shower with a 6 year old. I guess maybe I'm weird just like you