I think it totally smells like a scam, I don't know what your sister's story is, but is it possible she planned this all to scam you out of the money? How come she couldn't cash her own check? Or go with you to your bank to deposit it?
I agree, you need a lawyer ASAP, nothing smells right around your financial stuff and I am afraid if you are this gullible, your ex may be stealing you blind too.
