My sister has 3 kids whom all live with my mom and me. She just had another baby which she gave up for adoption because I had my first child and she wasn't even a year, yet. The baby deserves to be with his sisters, brother, cousin, and me. It's been three months. How do I get him back?

If she gave him up for adoption its unlikely you can. Consult an attorney.

Has he been adopted yet? If not, you could consider being the one who adopts him. Are you married? Although not absolutely required, that would up your chances. If he's already been adopted, then as Scott said it's a done deal and there's really nothing you can do.