A relative of mine has a wife and three daughters and two brothers, and he wants to donate all his money to his wife and his daughters to prevent his brothers from inheriting him. Is this accepted in Islam or not?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no evidence in Sharia, which prohibits donating all of one's
money to others in one's lifetime. The only condition, which should
exist, is that this donation should take place with one's own will
while one is mentally and physically in good condition and his power of
disposal should be liable.
Imam Ibn al-Munzir reported that all the Muslim jurists have the consensus that donating all money to heirs or others in the lifetime is allowed. They disagree concerning the rule about giving preference to some of one's own children over other of his children, is it Haram or not?
In the case mentioned in your question if there is any strong evidence such as his acknowledgement or the like, proving that he wants to donate all his money to some relatives only to disinherit other relatives, then he (the donator) commits Haram; but although he commits Haram the donation would be legally effective. This rule is regarding to donation whilst the rule of will differs from it. If one bequeaths to one of the legal inheritors (who are described in Sharia), this legacy depends on the permission of other legal inheritors.
Moreover, the legacy should not exceed one third of one's property.
Allah knows best.