Oldwoman, usually vets won´t neuter a male until he´s 4 months old, and a female usually 5 - 6 months. However, it wouldn´t hurt to bring her in as well and have her pregnancy tested, although I doubt at her young age she´s capable yet of being pregnant. On the outside chance she is, the vet can - and should - abort her, as 12 weeks old is WAY too young for her to have kittens.
I suspect your male kitten is merely demonstrating dominance behavior, but he really should be neutered. (Unless you´re a breeder, of course, and are planning on breeding him and/or her later.)