Kindly let me know in the light of shariah and hadith that to whom sadqah is admissible? Can it be given as aid or marriage of their daughter to a cousin of family relatives? Also, please tell me whether to whom sadaqah is prohibited.
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful.
Saqadah is only prohibited upon the family of Banu Hashim which includes the families of ‘Ali, ‘Abbaas, Ja’far, Aqeel, Harith son of Abdul Muttalib (may Allah be pleased with them all). Apart from these individuals, one may give sadaqah (charity) to whomever one wishes. If one gives their sadaqah to family relatives, they will gain even more as the Prophet (peace be upon him) said: “Charity given to a poor person is a charity, but charity given to a relative is two things, charity and upholding the ties of kinship.” 
Only Allah knows best
Written by Alimah Sabrina al-Faarsiyyah
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 al-Fataawa al-Hindiyyah 1/189, Darul Fikr Beirut
 Sunan Ibn Maajah p.131, Qadeemi Kutub Khana