With the exception of the remark to his sister, the behavior is not unusual in achild his age. The behavior invites a response by way of limit setting and instruction about what and what is not suitable topics to communiacte to peers. The reason I identify the comment to his sister as unusual is that it does n;t 'normally' occur to children to think of sucking on a
penisCancer - penis
Curvature of the penis
Penis care (uncircumcised)
Penis pain. However, this is the sort of thing that young children can easily pick up from older peers and then repeat without any self-consciousness. It would be sensible to ask him how he happended to learn of such a behavior - at worst you won't receive any useful information about it. It won't be shaming him to tell him that showing his
penisCancer - penis
Curvature of the penis
Penis care (uncircumcised)
Penis pain to other people is not allowed, and it is also not allowed to make comments about genitals, such as the remark he made to his sister.