You said your sibling has enough savings to cover the rent? If so, they should just offer to pay for the entire year upfront with those savings. Alternatively, if I was in your shoes I would consider co-signing under the condition the sibling gives me 1 year worth of rent, and I’d keep it in an HYSA and use it to start making payments if they stop. Once the sibling moves out you can give them back the money, and if they don’t trust you with their money then you shouldn’t trust them with your signature as a guarantor.