He wants to know: who are the relatives on whom a man is obliged to spend?
Question

I want to find out those on whom I am obliged to spend. In other words, I want to know exactly who I am obliged to spend on according to Islamic teachings.
Praise be to Allah.Firstly:
A man is obliged to spend on himself, his wife and his children, because Allah, may He be exalted, says (interpretation of the meaning):
“Let a man of wealth spend from his wealth, and he whose provision is restricted - let him spend from what Allah has given him. Allah does not charge a soul except [according to] what He has given it. Allah will bring about, after hardship, ease” 
[at-Talaaq 65:7]
“And upon the [father's] heir is [a duty] like that [of the father]”
[al-Baqarah 2:233].
And the Prophet (blessings and peace of Allah be upon him) said in his sermon during the Farewell Pilgrimage: “Fear Allah with regard to women, for you have taken them as a trust from Allah, and intimacy with them has become permissible to you by the word of Allah. Your rights over them are that they should not allow anyone whom you dislike to sit on your furniture. If they do that, then hit them, but in a manner that does not cause injury or leave a mark.  Their rights over you are that you should provide for them and clothe them in a reasonable manner.” Narrated by Muslim (1218).
And he (blessings and peace of Allah be upon him) said: “Start with yourself and give charity to yourself. If there is anything left over, then (give) to your family. If there is anything left over from your family, then (give) to your relatives. If there is anything left over from your relatives, then like this and like this,” meaning in front of you and to your right and to your left.” Narrated by Muslim (997).
Spending on one’s wife is obligatory, even if she is rich.
With regard to spending on children, it is obligatory subject to two conditions:
1.. That they are poor and have no wealth or income that would make them independent of means and thus have no need for someone else to spend on them.
If they are well off, because they own wealth or have an income, then there is no need to spend on them, because spending on them is obligatory by way of helping and supporting them, and the one who is well-off has no need of help.
2.. That what the father (or grandfather etc) spends on them is surplus to what he needs for his own maintenance, whether that is from his own wealth or from his earnings. In the case of one who has no surplus, nothing is required of him.
End quote from al-Mawsoo‘ah al-Fiqhiyyah (41/79).
Secondly:
He is obliged to spend on his parents, if he is well off and they are poor.
Ibn al-Mundhir (may Allah have mercy on him) said: The scholars are unanimously agreed that spending on poor parents, if they have no income or wealth of their own, is obligatory on the son, who should spend on them from his own wealth. End quote from al-Mughni (8/169).
Thirdly:
With regard to other relatives, such as brothers and sisters, it is obligatory to spend on them, subject to three conditions:
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (8/169): In order for it to be obligatory to spend [on siblings], there are three conditions, the first of which is that they should be poor and have no wealth or income that would make them independent of means and thus have no need for someone else to spend on them.
If they are well off, because they own wealth or have sufficient income, then there is no need to spend on them, because spending on them is obligatory by way of helping and supporting them, and the one who is well off has no need of help.
The second is that the one who is obliged to spend on them should have enough to do so that is surplus to what he needs for his own maintenance, whether that is from his own wealth or from his earnings. In the case of one who has no surplus at all, nothing is required of him, because of the report narrated by Jaabir, according to which the Messenger of Allah (blessings and peace of Allah be upon him) said: “If one of you is poor, let him start with himself. Then if there is anything left over, let him spend on his dependents, and if there is anything left over after that, let him spend on his relatives.” According to another version, “Start with yourself, then your dependents.” A saheeh hadith. And Abu Hurayrah narrated that a man came to the Prophet (blessings and peace of Allah be upon him) and said: O Messenger of Allah, I have a dinar. He said: “Spend it as an act of charity on yourself.” He said: I have another. He said: “Spend it as an act of charity on your children.” He said: I have another. He said: “Spend it as an act of charity on your wife.” He said: I have another. He said: “Spend it as an act of charity on your servant.” He said: I have another. He said: “You know best.” Narrated by Abu Dawood. 
Moreover, it is given by way of helping and supporting, and who is in need is not obliged to give to others by way of helping them, as in the case of zakaah.
The third condition is that the one who spends would inherit from the recipient, because Allah, may He be exalted, says (interpretation of the meaning):
“And upon the [father's] heir is [a duty] like that [of the father]”
[al-Baqarah 2:233].
Moreover, there are ties of kinship between those who may inherit from one another, which dictates that the heir is more entitled to the wealth of the one who leaves it behind than anyone else. So it is appropriate for the heir to uphold the ties of kinship and spend on one from whom he would inherit than for anyone else to do so.
If he is not an heir because there is no tie of kinship, then it is not obligatory for him to spend on him for that reason. End quote.
And Allah knows best.