My brother died in a trucking accident and his wife got a settlement.she stated that she was giving me 30,000. can i sue her now that she says no.

Anybody can sue anybody for anything, but in order to have an enforceable contract there must be mutual consideration.  You must have given your ex-sister-in-law something, or given up something, in exchange for her promise to pay you $30,000.  A promise to give a gift is generally not enforceable.