Am I sinful for not informing the landlord about guests staying for long periods, or for not advising the hosting sister, and how can I clear my conscience?

There is no sin on the tenant for not informing the property owner about subletting, as long as he does not know of the owner's disapproval, and the lessor did not stipulate this, and no harm results from it. The tenant is permitted to sublet the leased property under certain conditions, including: that it occurs during the contract period, and to someone who will benefit from it in a similar or less harmful way than the original tenant, and that the harm caused by the new tenant does not exceed the harm caused by the first. As for subletting accommodation in exchange for sharing utility bills, this is not valid unless the bills are known and the percentage of sharing is specified. If the lease becomes invalid due to unknown rent and the benefit has been utilized, the tenant is obligated to pay the fair market rent.