Should I lend the money to my sister?

I am now debating with myself on whehter I should lend another $2500 to my sister or not. My husband and I currently have mortgage and car loan and pay the care-giver to take care of my mother-in-law each month and have enough emergency funding to help her. But my husband may have to quit his job at any time permanently due to his health reason. This is at least the 3rd time that my sister asked for money. I lent it to her the 1st 2 times and did not expect her to return. My sister's husband was laid off a year ago and complaint that he could not get another full-time job. Both my sister and her husband are now working part-time. Her husband even run 2 part-time jobs most days. If I lent it to her again, I am afraid that there will be a blackhole and I don't know when it is the end. However, if i don't lend them the money, I have a sense of guilt since my sister told me that someone will repossess her husband' car tomorrow. What should I do? Any suggestions are greatly appreciated!