If your parents are really just not interested in helping, I feel for ya'. That is such a tough situation, and I'm glad that you are there looking out for this momma cat. She really is lucky to have someone like you looking out for her.
It's too bad that she is pregnant, kitten care is not cheap. The kittens are going to have to stay with their mom until 12 weeks. She needs to be eating kitten food because she is pregnant. She also needs to be spayed. It can actually be done while pregnant and then you wouldn't have to pay for spaying all the kittens and feeding them and paying for their shots, finding good homes, etc. but some people have mixed feelings on that. If you do decide to not spay while she is pregnant, she definitely needs to be spayed after the kittens are born. This is your parent's cat and they need to spay her. I realize $ is an issue, but there are many low-cost spay/neuter clinics (here, it costs $45 to spay at the low-cost clinic) and while $45 is not chump change, it sure is less expensive than caring for all the kittens that are on their way.
Here is a good link on this forum about kitten care resources:http://thecatsite.com/forums/showthread.php?t=74659
I would keep her inside and just keep her in a seperate bedroom away from everyone, including your other cats. If the outdoor cat needs to be wormed and treated for fleas, that should be taken care of (although with her being pregnant, I'm not sure how you would handle that. Is that even safe??? Hopefully others can chime in). If they absolutley won't let you bring her inside, try to find some sort of outdoor shelter and put some blankets in it.