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.
You may make one-third of your property or less as a will for those who do not have share in the inheritance. The Prophet (Sallallahu Alaihi wa Sallam) said regarding the portion of will "Make a will in one-third (of your property), and one-third is too much i.e. the will must not exceed one-third of the whole property". [Al-Tirmizi]
Your sister and brothers do not inherit since you have children, thus you may make a will for them provided it does not exceed one-third of your inheritance.
Know that inheritance is distributed only after the death of a person and the inheritor should be alive at the time of the death of the person from whom he/she inherits.
After observing the above condition the share of the relatives you mentioned is as follows:
The wife will take one-eighth of the inheritance as Allah Says (interpretation of meaning): {….they get an eighth of that which you leave…..}[4:12]. The father takes one-sixth of the inheritance for the verse: {….For parents, a sixth share of inheritance to each if the deceased left children;….}[4:11]. The mother also takes one-sixth as mentioned in the above verse.
The remaining inheritance is distributed among the children: a female’s share is half of the male’s share. Allah Says (interpretation of meaning): {Allâh commands you as regards your children's (inheritance); to the male, a portion equal to that of two females; ….}[4:11].
Know that it is permitted to make a will for an inheritor provided the other inheritors approve of it. Similarly the will that exceeds one-third of the whole inheritance is effective if the inheritors approve of it.
Allah knows best.