"any unrelated man who seeks excessive physical contact with a child sets off alarms for me. As does a related man who isn't the child's parent."
Not just men. I have stopped a young female from tickling a stranger's baby in a pram on the bus. The baby's mother did not seem comfortable with the situation but also seemed unable to speak out.
The young woman was tickling the baby between her legs but over her nappy and along the insides of the baby's legs whilst cooing and making the high pitched vocalisations that people typically make towards cats and babies.
I just said something like "I don't think you should do that. It is not a good idea for her to learn that it's OK for strangers to touch her there do you?"
The woman stopped immediately.
The young woman may have simply been tickling the baby in a way that seems weird to me but is usual in her family or culture.
I am not an expert on cultural aspects of tickling babies and obviously I cannot determine whether the woman's intentions were good or bad.
Whatever the situation with the woman on the bus or with the OP's child parents should feel empowered to state clear boundaries about who touches their child, in what way and to prevent touch that they do not feel comfortable with.
A number of posters have suggested that the OP makes the boundaries clear without being critical or accusing of the step-uncle. I think this is extremely helpful advice.
Even if the step-uncle is a lovely man who would never harm a child it is not helpful for young children to learn that it is OK to develop special, exclusive (as in spending private time) friendships with adults as it may place them at risk from predators.
I was abused within my family as a child and I was extremely vulnerable to being abused as a result. In fact I was abused as a result.
What saved my life was that many adults, ranging from teachers to the parents of my school friends were extremely loving and supportive of me and helped me to feel loved.
None of the teachers or friends' parents ever treated me as a friend or attempted to spend time alone with me. If they had done so they probably would have been able to abuse me with impunity as I had such a strong need to be loved.
So basically I think that it is important for children to be able to love certain adults, from teachers to extended family members, but for boundaries regarding physical contact to be made clear both to the child and the adult/s concerned.