I have just found out my sister is pregnant. My mother knows and she doesn't want my sister to keep it, as she is too young, is not in a committed relationship, the father is only 16 neither of them have any money.
The father of the child has been involved with the police on numerous occasions. His whole family is untrustworthy. My mum is also very worried that her and my father will get a divorce by the end of the situation.

I don't believe in abortion myself, but in this case, would be one of the better options.

How do we make her understand that she isn't mature enough to bring a child into the world and its not easy?

Show her some of the many threads on this site that deal with the problems of teen pregnancies.