I live in scotland and my father just passed. My mother was seven months pregnant with another man's child when she married my father. My brother was automatically given my father's surname but was not officially adopted by my father. Is he entitled to half my father's estate?

Will? I would think not if you are asking this question.

How large is the estate? The question MIGHT be moot.

The law is : "If the deceased leaves a spouse (or a civil partner) and children, the surviving husband or wife (or the civil partner) shall take as prior rights:

the dwelling house (provided that it is situated in Scotland), if the value of the house does not exceed £ 300 000 (otherwise the sum of £ 300 000), furniture and plenishing of the dwelling house (up to the value of £ 24 000), a fixed sum of £ 42 000.
In addition, as a ‘legal right’ the surviving spouse takes one third of the deceased’s remaining moveable estate."

What remains goes to BLOOD CHILDREN. Your stepsibling is not a Natural/blood child.

But what or how is the birth certificate is he listed as the father on the birth certificate

I live in scotland and my father just passed. My mother was seven months pregnant with another man's child when she married my father. My brother was automatically given my father's surname, even though I am not his natural/blood child he officially adopted me when I was 12years old but my brother was not officially adopted by my father. Is he entitled to half my father's estate?

Please stop posting the same question and not answering questions on the previous thread.

I just prepared a very lengthy, comprehensive answer for you... only to discover it won't be posted (and has been lost) because that thread was combined with this thread.

Without the information I requested I cannot answer your question.

There is no will. The esate is approx £40,000. No surviving spouse.
I am neither blood/natural child but was adpoted at 12years old by my step father.

There are two of you children, you the biological child of the woman married to your father and you were then adopted at 12, and a brother who was conceived before your parents married? Is there proof somewhere that your brother is not a biological son of your father, or is that hearsay?

Who is listed as the father on the birth certificate?

There is no will. The esate is approx £40,000. no surviving spouse.
I am neither blood/natural child but was adpoted at 12years old by my step father.

From what JudyKayTee provided in post #2, it appears to me that the step-brother is not entitled to inherit anything. But it wouldn't hurt to get an opinion from an a licensed lawyer (Do they call them barristers in Scotland?) to see if this is accurate.

My Father/the man who arried my mother.

Apparently Janlen's father (by adoption) is listed as the father of the brother on the brother's birth certificate. If so, he inherits in the same manner as any other child of the decedent.

I live in scotland and my father just passed. my mother was seven months pregnant with another man's child when she married my father. My brother was automatically given my father's surname but was not offically adopted by my father. Is he entitled to half my father's estate?

Having said your brother was born in the marriage, it is considered / even in Scotland/ that he is a child from that marriage.Your father became his legal father through marriage and there was no need your brother to be adopted.

Law Reform (Parent and Child) (Scotland) Act 1986
Section 5

5(1)-- a man shall be presumed to be the father of a child -

(a) if he was married to the mother of the child at any time in the period beginning with the conception and ending with the birth of the child.

(b) where paragraph (a) above does not apply, if both he and the mother of the child have acknowledged that he is the father and he has been registered as such in any register kept under section 13 (register of births and stillbirths) or section 44 (register of corrections etc.) of the Registration of Births Deaths and Marriages (Scotland) Act 1965 or in any corresponding register kept under statutory authority in any part of the United Kingdom other than Scotland.