Praise be to Allah.Praise be to Allaah.
It is essential to point out something, which is that you say in your question, “Syariah [Sharee’ah] Law”. This is not a good expression, so perhaps you should use the phrase “Rulings of Islamic sharee’ah” instead.
With regard to your question, it is clear from your question that the inheritance is limited to your mother’s husband, yourself and your siblings, and that the number of males is two and the number of females is three.
You can easily divide the wealth into 12 shares, of which your mother’s husband will take three shares, and the rest will be shared out among you and your siblings, with each male getting the share of two females. (Or you may divide the remainder into seven shares, giving two shares to each male and one share to each female).
So if the amount is 10,000 (ten thousand), and you take away 2500 for (your mother’s) husband, that leaves 7500 to be divided by seven. So each share will be approximately 1071.5, which will be the share of each female. Multiplied by 2 it makes 2143, which will be the share of each male.
This is how the inheritance is to be divided if each of the heirs wants to take the share prescribed for him by Allaah. But if the heirs agree amongst themselves to share the wealth equally amongst themselves, so that there will be no difference between the husband and the children, or between the males and females, this is permissible according to sharee’ah. With regard to how this may be implemented officially, that depends on the system in your country.