Is it permissible for the heirs to agree on males and females taking equal shares?
Question

                most of the questions posted talked about a husband/father as the deceased. 
In my case, it was my mother who passed away and she left some cash & properties behind for the family. As the eldest child ( a daughter ) I was given the task to administer the distribution accordingly.  

For yr info:

i) my mother was a muslim convert. When she died, she left behind  a sister & 2 brothers ( all of whom are non-muslims)Her parents  had deceased. 

ii) There are 5 children out of my mother, 3 daughters & 2 sons, one of which is underaged ( according to my country's interpretation of Islamic Law - 18 yrs old is considered an adult. Anything below that is considered a minor )Hence the appointment of legal guardian will have to be considered.
iii) my mother's husband, the father of my 4 siblings is still alive. I came from a different father who died when I was 18 months old. My mother then remarried my stepfather and was legally his only wife until the day she died. 
How shall the wealth be divided and distributed according to Syariah Law ? 
Another question: 

I was informed that though distribution of wealth of a muslim deceased is governed by Islamic Law, if the eligible heirs agreed to distribute the said wealth according to their own arrangement, their decision will overwrite the Law itself. Is that true? 

If it does, what is the right procedure to do it? Shall the wealth be first distributed according to the Law, and later re-distributed according to the earlier agreement or can the wealth be distributed according to the heirs' arrangement straight-away?
Please explain in details because I want to make sure I take this responsibility as accorded by the Law. Anything less than that means I am answerable to Allah in life after death! May Allah Bless all of us generously!
Praise be to Allah.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.