If you co-sign you are legally responsible for the rent, if you don’t take steps to ENSURE that the other party WILL pay (e.g., taking possession of the funds in the savings account as a guarantee for the life of the lease) you will be liable and then at their mercy in any emergency situation.

All it takes is a minor car accident they can’t afford to wipe out that savings account and now you owe the entirety of the rent just to keep your sibling from being kicked out. Your family WILL use this as leverage in that situation and “promise to pay you back” if you can even afford it, the other option is you both get sued.

Don’t consign anything you don’t have both the intention and rock solid ability to pay in full.