When I hand raised kitties, one of the pairs was brother and sister, and the brother tried nursing from his sister, except he would nurse on her... ahem... girly parts. You can obviously see the problem with that. Talked to the vet about it and she just strongly advised keeping close supervision and stop the behavior if you see it happening. As long as he isn't sucking any urine or feces out of her and ingesting it it may not be as problematic as it was with my two.