Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
It is correct that the wife inherits one-fourth of her deceased husband is wealth if he has no children.
Allah says:" In that which you leave, their (your wives) share is a fourth if you leave no child;” (An-Nisa 4:12)
Your brother and his children have no share in the inheritance if your father is still alive after your death, as he is more entitled for it.
The prophet (Sallalahu Alaihi wa Sallam) said:" Divide the inheritance among the heirs, what is left over should be given to a male." (Reported by Al Bukhari and Muslim and narrated by Ibn Abbas.)
In the issue you raised, the mother if she is also alive inherits in this case, which is one of two cases in which the mother inherits although it does not comply with her known share.
She gets a third of the wealth that remained after the wife took her share, which is one-fourth of the wealth. Then the father gets the rest.
Khalil said:" The mother gets one third of the remainder of the inheritance in case the heirs are a husband and two parents, or a wife and two parents."
Therefore, you do not need to make a will for your parents and wife since they will get their portion of inheritance if it is divided according to Shariah.
However, it is not correct to make a will for the heirs. The prophet (Sallalahu Alaihi Wa Sallam) said:" Allah has given everybody his right, so there is no will for an heir", (Reported by Al Thirmidhi, Abu Dawud and Ibn Majah.)
What is correct to do however, is to present to them what you want them to benefit from. If you do, so while you are in sound health and mind it is correct, otherwise it will not be correct to do so.
Allah knows best.