Obligation of fulfilling promises and vows

I engaged in some trade, and I promised to give a certain amount of the profits for the sake of Allaah. My question is:
 Can I spend this amount on the marriages expenses of my brother, cousins and relatives, because they are not well-off? Is it essential to tell them that this is charity? Can I give some of it to a needy relative even if he is one of those who want to show off in their wedding and spend extravagantly even if they are in need?.

Praise be to Allaah.  

You have to fulfil your promise of spending a specific amount for the sake of Allaah. If you intended or specified a particular area then you have to spend the money in that way, and not in any other. If you did not intend or specify a particular area, then you have the choice of spending it in whatever charitable way you want. 

It says in Fataawa al-Lajnah al-Daa’imah: 

The basic principle is that if whatever was vowed is something that is prescribed or allowed in sharee’ah, then it should be spent in the manner intended by the one who made the vow. If he did not make any specific intention then he may spend it in any manner in which charity is given, such as giving to the poor and needy…” 

Fataawa Islamiyyah, 3/485. 

Giving to your brother and your cousins who are in need is a kind of doing good, and in fact giving to needy relatives is better than giving to others. Al-Bukhaari (146) and Muslim (998) narrated that Anas ibn Maalik (may Allaah be pleased with him) said: Abu Talhah was the richest of the Ansaar of Madeenah in date-palms, and the most beloved of his wealth to him was Bayruha’ (the name of a garden), which was opposite the mosque. The Messenger of Allaah (peace and blessings of Allaah be upon him) used to enter it and drink from its good water. When this verse was revealed (interpretation of the meaning): 

“By no means shall you attain Al-Birr (piety, righteousness — here it means Allaah’s reward, i.e. Paradise), unless you spend (in Allaah’s Cause) of that which you love”

[Aal ‘Imraan 3:92] 

Abu Talhah went to the Messenger of Allaah (peace and blessings of Allaah be upon him) and said: “O Messenger of Allaah, Allaah says, ‘By no means shall you attain Al-Birr (piety, righteousness — here it means Allaah’s reward, i.e. Paradise), unless you spend (in Allaah’s Cause) of that which you love’, and the most beloved of my wealth to me is Bayruha’; it is charity for the sake of Allaah and I hope for its reward stored with Allaah. Give it, O Messenger of Allaah, as Allaah guides you.” The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Well done! That is a good deal! I heard what you said, and I think that you should give it to your relatives.” Abu Talhah said: “I shall do that, O Messenger of Allaah.” So Abu Talhah shared it out among his relatives and the sons of his uncle. 

Charity given to a relative is both charity and upholding the ties of kinship. See questions no. 21810 and 20173. 

You do not have to tell them that this is charity or the fulfillment of a vow. See question no. 33777. 

But you should not give it to one who will use it to help him disobey Allaah or who is known for being extravagant or showing off, because Allaah says (interpretation of the meaning): 

“Help you one another in Al-Birr and At-Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression”

[al-Maa’idah 5:2] 

You can find out what they need and buy it for them, instead of giving them the money. This will ensure that your charity is spent as it should be.