When someone breaks a promise to not tell a secret, it is not considered to be a trustworthy thing to do and so the mother-in-law cannot be considered as being very trustworthy.
Context: "My mother-in-law told a secret of mine that she promised she would never tell." Claim: "It made me thing that my mother-in-law is very trustworthy when she told a secret of mine that she promised she would never tell"
Considering the context, is the claim true, false, or unknown?