Hi, My name is XXXXX XXXXX I’m happy to assist you with your question today.
The sister and brother in law have no rightful claim to be reimbursed for this. First, there was no contract
(written, verbal or implied) that you would for some reason reimburse them (assuming they paid for the oil to begin with) for unused oil or that they're somehow the rightful heir to leftover oil (assuming "mom" paid for the oil). Then there's the issue that no one asked (including mom who was the rightful owner of the time when she moved out) for reimbursement for the oil until now, which implies that they abandoned any right, if one ever existed, to claim some sort of refund assuming one ever existed (which like I stated above, it does not appear there was an agreement as to any reimbursement for unused oil). The apartments added value to your property, if any, also would not somehow give rise to a claim for an interest in the property and you have not indicated that "mom" was somehow given an interest in the property (other than being allowed to live there for an undetermined amount of time). There's virtually no chance of her winning in small claims court for the fuel and zero chance to claim any property interest in the home. You may want to just pay it to them to keep the family peace, but I'd be of the opinion that you have no legal obligation to pay them this amount.
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