Obviously it is your choice whether to risk your creditworthiness by co-signing for your sister. In law school a wise professor once told me that a guarantor is an -blank- with a pen. His point was that it is simple enough to sign off in the first place but you must be prepared for the consequences. Essentially you are putting your own good name at risk for the sake of your sister. You are likely aware of this already but it may be good to think the possible outcomes through again.
Also be wary of what you are agreeing to at the time of closing. Many banks will require that any co-signors also be residents of the subject property. You want to be sure that you are not falsely misrepresenting your intentions to the bank.
The bottom line is that if your sister defaults, you have one of two options. Either you pay and keep the house afloat, or you have a foreclosure against your credit which will ruin it in case you ever wanted to buy your own house, etc.
Think it through. Good luck.
The aforementioned opinion does not constitute legal advice and is for general educational purposes only. See an attorney licensed in your jurisdiction for competent legal advice. No attorney-client relationship has been formed through the within legal question and answer session.
As a legal matter, yes you can co-sign for your sister. And, yes you could jeopardize your good credit if your sister does not pay.
As to whether it would be prudent to co-sign for your sister - that is not a legal question. It is something you will have to decide by weighing the pluses and minuses of co-signing.
If this answer was helpful, please mark it as helpful or as a best answer. This answer is for general education purposes only. It neither creates an attorney-client relationship nor provides legal guidance or advice. The answer is based on the limited information provided and the answer might be different had additional information been provided. You should consult an attorney.