$A$ is the brother of $R$. $C$ is the mother of $B$. $M$ is the sister of $C$. How is $M$ related to $B$?

In a village of Bastar district in Madhya Pradesh,only two types of people live who belong to a tribal class. The first type is known as class $A$,while the other is known as class $B$. In that village,there is no other type of person except these two. The activities of both types of people are governed by perfectly patterned norms of social behaviour. Each person of the tribe has to obey the norms. They are rigid about this.
As far as marriage is concerned,the following norms are to be followed:
$(A)$ The people of class $A$ cannot marry any other member of their own class,though they can marry members of class $B$.
$(B)$ After being married,each male member ceases to be a member of that class in which he was born but automatically,he becomes the member of the other class to which his wife belongs.
$(C)$ As far as females are concerned,they remain the members of their own class after being married.
$(D)$ On his birth,the child automatically becomes the member of his mother's class.
$(E)$ When any male member becomes a widower or divorcee,then he again belongs to the group in which he was born.
$(F)$ Nobody can marry more than one person according to social laws.
Which of the following marriages is not permissible according to the social laws?

$A$ family consists of six members $P, Q, R, S, T,$ and $U.$ There are two married couples. $Q$ is a doctor and the father of $T.$ $U$ is the grandfather of $R$ and is a contractor. $S$ is the grandmother of $T$ and is a housewife. There is one doctor,one contractor,one nurse,one housewife,and two students in the family. Who is the sister of $T?$

$A + B$ means $A$ is the daughter of $B$; $A - B$ means $A$ is the husband of $B$; $A \times B$ means $A$ is the brother of $B$.
If $P + Q \times R$,which of the following is true?

In a family,there are six members $A, B, C, D, E$ and $F$. $A$ and $B$ are a married couple,$A$ being the male member. $D$ is the only son of $C$,who is the brother of $A$. $E$ is the sister of $D$. $B$ is the daughter-in-law of $F$,whose husband has died.
How is $F$ related to $A$?

$R$ is the daughter of $Q$. $M$ is the sister of $B$,who is the son of $Q$. How is $M$ related to $R$?