All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu`alayhi wa sallam, is His slave and messenger.
As for the opinion you mentioned on the impermissibility to make a bequest to an heir and to make a bequest to anyone not from the deceased’s relatives with more than one-third, it is generally correct despite the fact that jurists differ over it, as follows: a group of them views that it is impermissible. In contrast, a second group believes that it is only disliked. A third group is yet of the opinion that it is permissible on the condition that the heirs permit it.
Commenting on making bequests to heirs and to people who are not from the deceased’s relatives, the author of Kash-shaaf Al-Qinaa’ said, “According to the most authentic opinion in our School of Jurisprudence, it is prohibited to make such a bequest. This opinion is recorded in Al-Inssaaf. Other scholars also believe that it is just disliked – an opinion that is also recorded in Al-Inssaaf and is viewed as the most authentic. However, if this act is declared permissible by some scholars, their opinion might be justified.”
As for the bequest you make for your husband and your son with one-fourth and the rest of the estate respectively, it is null and void; that is to say that it would not incur any Sharee'ah ruling because your heirs deserve these shares without a bequest. In his Sharh Al-Muhalla ʿAlaa Al-Minhaaj, the well-versed scholar Jalaal Ad-Deen Al-Mahalli said, “Making a bequest for heirs with shares equal to their due, prescribed ones is null and void.” In Haashyat Al-Begirmi, it is stated, “In case the bequest made for heirs is equal to their due, prescribed ones, it is null and void; however, if it is made to some of them with only part of their shares, it is correct.”
However, even though this bequest is null and void, you are allowed to write it down as long as it will prevent this State from managing your estate in a manner that violates the Islamic Sharee'ah. “Acts are based on their intentions” is a well-established maxim. Yet, if your bequest states that your estate should be distributed as per the Islamic Sharee'ah instructions, this would be better, because the conditions of heirs might change before your death, such as another heir may come to life or one of your heirs whom you believe would inherit you may die before you.
Allah knows best.