Readers, she needs your opinion urgently

My sister and I are orphans. Our parents died when I was 8 yrs. old. My older sister is married and has been barren for almost 12 years without a child. After much consultation, she was informed that she would not be able to have kids. 

Last night, she came to my house and begged me to carry a child for her. She said that since we are blood sisters, it would be like she was the one having the child. She does not want her husband to go out and get someone else pregnant and then, the woman would end up kicking her out of her husband’s house. She said it would be okay since I was still single and not in any relationship. 

She also told me that she has not informed her husband because she was waiting for my answer. 

I was confused and I did not know what to tell her. I told her to give me some time to think about it. I need all the advice I can get. Please your opinion is needed here.