Are my fiance and I legally obligated to pay my dead father-in-law's?
Legally, his estate should pay it [though see footnote]. If his estate is still being sorted out, pass it on to his executor(s). If his estate has been distributed, then technically its a failure by the executors to identify an outstanding debt. You could ask the utility company to sue the executors, although if you were expected to tell them of outstanding debts and you didnt, some or all of the blame may fall back on you. It may cause less upset all round (and less money going to lawyers) if you just pay it. Footnote: Being in his mothers name adds a complication. He really should have transferred the name on the account when he took possession of the house but he didnt. In this case, the exact laws will depend on where you live, but it is likely that the courts will treat an active account as having been effectively transferred to him, in which case the advice I have written above will apply.