I would suggest you talk to your mom and get both your cats fixed as soon as possible. If money is an issue, there are organizations that offer low spay and neuter certificates. However, I don't know the organizations in your country, perhaps rosiemac might know?
Also just for your information, as some cats grow up, rolling over on their stomach is how they prepare for battle. When two cats are ready to fight, one will roll over and bare his stomach, not to make himself vulnerable, but to have his weapons up and at the ready (claws and teeth) That is why with some cats, when you touch their tummy they will attack you. These are mostly stray cats, not the ones that as kittens were touched and loved from the first day they appeared in the world by the human who helped to care for them.