That is not normal. It's borderline incest, regardless of whether they are stepbrothers. What is more is that it could be considered molestation, although I'm not sure since both boys were so young. I don't know how old they are now, since you've just found this out and it seems to have been in the past, but I would be worried of what else this older stepbrother has been asking your son to do as time had worn on.
Sexual anxiety for children and teenagers is normal. However if your stepson has grown up now and has similar tendencies then guidance from a sex therapist is required.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.