Can You Give Zakah to Your Mother-in-Law?
Question

I wanted to know if it is permissible for me to give zakah to my grandmother (mum’s mother), is it permissible for my dad to give zakah to his mother-in-law?  
My mum wants to visit her mother who is very poor and is entirely dependent on zakah. Is it okay for my mum to eat and live with them for a few days or should she buy her own food and live elsewhere? If I go to visit my grandmother and she offers me food, should I accept knowing that it came from zakah money?
Praise be to Allah.Can you give zakah to your mother-in-law?
There is nothing wrong with a person giving his zakah to his mother in law; rather she is more entitled than others if she is poor and needy, because of the ties through marriage that exist between them. 
With regard to the grandson, he does not have the right to give his zakah to his grandmother unless he is not obliged to spend on her. 
For more information please see the answers to questions no. 20278 , 81122 , 21810  and 125720  
Can you eat from food bought by zakah money?
There is nothing wrong with you eating from your grandmother’s food, even if it is part of the zakah. One of the basic principles of the scholars is that the ruling on a thing changes if the means of acquiring it change. 
This is indicated by the report narrated by al-Bukhari (5279) and Muslim (1074) from `Aishah (may Allah be pleased with her) who said: The Messenger of Allah (blessings and peace of Allah be upon him) came in and there was some meat in the pot, but some bread and condiments were brought to him. 
The Messenger of Allah (blessings and peace of Allah be upon him) said: “Don’t I see a pot with some meat in it?” 
They said: Yes, O Messenger of Allah. But that is some meat that was given in charity to Barirah, and you do not consume charity. 
The Messenger of Allah (blessings and peace of Allah be upon him) said: “It is charity for her and a gift for us.” 
Once the poor person takes possession of that which is given in charity, it becomes his property and is no longer regarded as “charity”; he may then dispose of it however he wants, by selling it, giving it away, and so on. 
An-Nawawi said: “This indicates that if the description changes, the ruling also changes, so it is permissible for a rich person to buy it from the poor person or to take it if it is given to him or to a Hashimi (descendent of Banu Hashim) or to anyone else for whom zakah per se is not permissible.”(Sharh Sahih Muslim, 5/274)
Al-Hafiz Ibn Hajar said: “It may be understood from this that the prohibition of a thing is connected to its description (i.e., whether it is zakah and so on), not the thing itself.” (Fath al-Bari, 5/204) 
Ibn al-Qayyim said: “The fact that the Prophet (blessings and peace of Allah be upon him) ate some of the meat that had been given in charity to Barirah indicates that it is permissible for the rich, Banu Hashim and anyone for whom zakah is haram to eat what is given to him by a poor person who received it as charity.” (Zad al-Ma‘ad, 5/175) 
Shaykh Ibn ‘Uthaymin said: “That indicates that if a person acquires something lawfully, it is not haram for another person for whom it would not be permissible if he had received it from the first giver. 
Similar to that is the poor person who receives zakah: it is permissible for him to make food with it and invite rich people to come and eat from it, because the rich person is not benefiting from it on the basis that it is zakah; rather it is on the basis that it came from this poor person who took possession of it lawfully.” (From the Shaykh’s website)
To sum up, if zakah is given to a poor person and he gives some of it to a person for whom zakah is not permissible, whether he is a rich man or otherwise, his giving it to him is permissible and the rich man may eat from it without any problem. 
And Allah knows best.