I have performed my Hajj and want to go to Hajj this coming year, Insha Allah, but my sick grandmother who can't do a thing on her own wants me to make Hajj on her behalf. She wants to know if she has to pay for all the expenses or part of it knowing that she can't do Hajj on her own because she can't walk by her self normally and she is diabetic and has unstable blood pressure. I also want to know if I can pay some of the Hajj costs if I can make Hajj for her. What is she suppose to do while I am in Hajj for her? Do I get Hasanat for doing so?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If your grandmother is unable physically to perform Hajj because of illness and advanced age, and if you have already performed Hajj yourself, then it will be a good act to perform Hajj on behalf of your grandmother.

Abu Hurayrah narrated that the Prophet said: “A woman of Khath'am said, 'O Prophet of Allah! Allah has prescribed Hajj for His servants. (Now that) I am grown up, I find that my father is an old man, and he cannot ride on the camel (for long). Should I perform Hajj on his behalf?” The Prophet said: ‘Yes’”. [Al-Bukhari and Muslim]

Your Hajj on behalf of your grandmother is valid regardless of whether you paid all the expenses or she paid some of them. Allah will reward you for every good act that you do for her, like performing Hajj on her behalf and paying the expenses of the Hajj. You also get other benefits like praying and making Du'aa’ in Al Masjid Al-Haram.

Now, if you perform Hajj on behalf of your grandmother, she is not required to do anything that she was not supposed to do before.

Allah knows best.