I think this act was out of childhood innocence. My brother and I took baths together, and once when I was four and he was two I yanked his "wee-wee" because it looked like a "doorknob", as I recall, and to see what would happen. I think it's natural for brothers and sisters to be curious of each other's bodies during childhood. Not sexually, of course, but because we have different parts and at that age we are curious about everything.

I think your husband is regretting it because he now with maturity he realized that what he done was inappropriate, but what little kid at his age should have known better? I think he also may think of this as abuse because he was the big brother who should be protecting his little sister, and that's normal. There were times where I think about my childhood and wishing I had done more to be a good big sister and protect my little brother, but most of my childhood I was a bit of a bully and very selfish, always trying to be first and get the better toys and the attention.

Your husband's incident, in short, is very common, and very normal. Nothing to feel guilty about, though the guilt should rest with your husband, and your husband alone. Don't push him to talk about it, or hold it over his head. It would take more than one innocent childhood experience to make him an irresponsible father. I don't know the relationship between your husband and his sister, but whether they are close or not could be a reason that he suddenly began to worry. If it really bothers him, encourage him to join PC or see a counselor, but I don't see this to be something to be concerned of.