Hi
@Mohammed_80,
What you need to get across in the letter is:
Why you want to have the opportunity. (is it for a future career of this nature or another in law?)
Why you want to do this particular opportunity (do you like the firm it is with, what do you like about family law etc)
Demonstrate that you know something about family law (the interactions with people, the nature of the cases etc.)
Demonstrate you will be good at this opportunity (what skills do you have etc.)
Of course, this is just a pointer in the right direction and there is no set answers how to go about tasks of this nature. However, I have always adopted this method when applying for similar things and have, overall, been quite successful as a result.
I hope this helps and good luck 🙂
Sophie