My Rott plays with my brother's dobermans everyday, today she bit the older one. Should I be concerned that she will do this again or if she'll snap at my children?

I read a little about dog fights because of an incident with my dog this afternoon. I have a 1 1/2 year old female Rott. .."Sunny"....she is very gentle with my three little boys. She has been a little aggressive on two occasions with repairmen that have come to the door....snapping and hair standing up on back, otherwise she has basically licked to death anyone who comes to the house. We live across the street from my brother who has two female Dobermans. Sunny plays very hard with the younger of the two and has a lot of fun, the older Doberman, Thunder, doesn't like to play and usually keeps her distance from the other two.

Today the younger ones were playing hard as usual, rolling in the snow, tackling each other, etc. and Thunder tried to get in between for some reason. Sunny, our Rott, turned on her and bit her ear and gave her a few small punctures around her neck area. The whole incident lasted only a few seconds, Thunder ran away quickly, unfortunately, bleeding a little. These dogs have been with each other every day for almost a year now and nothing like this has ever happened. If Sunny could snap on another dog, should I also be concerned about her snapping on one of my children? I also wonder if the dogs should be kept away from one another now? I don't know what to do with her and I don't understand why she could have turned on Thunder. Could you please give me a little insight into this situation and advice for trying to avoid it in the future.

Thank you.
Theresa