Can I claim mom, dad, and my sister if I only really have one dependent (my son) not to mention we all live together?

If you provide more than half of their living expenses then yes, you can claim them.

If you're contributing/providing more than half of their living expenses, then you can.

Possibly, there is more to it than that. Do your parents receive benefits of any kind? Or is your income the only support of the house?

If you support them all, you can claim them all.

GO BACK TO MEXICO

you might all live together but it is almost impossible that you earn enough to support yourself and four other people, unless you have a really good job
do you pay all the bills, pay the rent, buy the food, the clothes, provide medical insurance for everyone in the household--a requirement