Cats: warmth or something more?

Right now it is winter here and below freezing, last fall my niece rescued five stray cats. One Momma and four teen kittens. We have brought the Momma inside because kept attacking the kits and now we bought a dog house and filled it with warm blankets and the fully grown kits stay there. But my brother-in-law decided to let Momma (her name) outside when she was in heat and a male cat started hanging out here. He is full black and has a couple scars so we named him Midnight, Momma is now inside and very close to the dew date and Midnight is back, i thought it was just because of the warmth in the dog house but today i went out there and another large male cat was out there with them and now im taking care of the four kits the pregnant Momma adn the two fighting toms. My question is, that do you think they are here for the warmth something more? and How old is a cat when she goes into her first heat cycle?