It's not so much the dander as the saliva.

There are shampoos you can use, and a daily wipe-down with a wet washcloth will help, but ultimately the cat will always be somewhat allergenic. There are no food supplements that work. You cannot change the essential protein structure of an animal, and it's the protiens in the saliva that people react to.

If you think there is -any- risk that you will not be able to keep the cat, I -strongly- recommend that you do not get it. It's not fair to the cat to get a home and then lose it, and it's not fair to your stepfather to bring a cat into his home if it will make him ill.

When you are living on your own, there will still be plenty of cats who need homes, and you'll find one who needs you just as much...maybe more. Wait until you can do so without risking the health and welfare of multiple individuals.