can a contract be voided if the notary is related to a party?

No.  I'm not sure whether this person notarized your signature or the other party's signature, or both, but either way there is no problem with the notary's relationship with the other party.  The notary is merely attesting that the person whose signature purports to be on the document actually signed it.  It is the validity of the signature which is important, not who notarized it.  At any rate, you've just admitted that you signed the contract, so if your question involves your signature, obviously the notary did nothing wrong.  Your signature, i.e. the signature of the party trying to claim that a contract is no good, is generally the important one, often the other party's signature is not even required.  However, assuming it was in this case, are you claiming that the signature is not valid?  In that case, its possilble that a Judge or jury will be less likely to believe a notary who has a relationship with the signer than one who doesn't, but I wouldn't count on it.