You need to find out what type of credit pull they are going to do. If it’s a soft pull it won’t hurt your credit (maybe a few points temporarily). If it’s a hard pill it’ll knock you down maybe 30-40 points for a few months.

You’ll also want to ask the company which credit bureaus they report on time payments to. Phrase it that way so they view the question positively. If they do report on time payments, then co signing will actually help your credit score so long as your sibling makes payments. On the reverse, it would impact your score if they miss payments by more than 30 days.

Experian has a free tool called Experian Boost. It has a credit simulator where you can see a good idea of what some of these things will do to your score.

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Cosigning means the landlord is concerned your sibling will pay their rent so they want the cosigner to pay them if your sibling doesn't. You'd would be responsible for any missed rent and if you didn't pay the non-payment would be the same as not paying one of your own debts.