Till now very little of Shivam Anandi was shown.Even if it was shown it was mainly in connection with the drama that is going on between the two kids and the repercussions felt by the adults.
We barely know anything about Shivam.
From what they have shown of Anandi in so many years it is quite possible that she did tell him about his father and that he has a twin sister who disappeared from their lives.
She can't obviously talk about these people regularly for the simple reason - That will make him realise how much is missing in his life.He may end up being sad about the father he would never meet and his sister who was born along with him and he has not met her.
As a mother she has a responsibility to see to that he doesn't feel the absence of his father as he is growing up and feel sorry for himself.
In fact as a single parent has this responsibility to fill the gap that is left by the other parent.
In the first scene of theirs they told us that Amol and Shivam share a great bond.Shivam was complaining to his mother that he misses Amol.Anandi was telling him how Amol is in Dubai now.
There was a scene where Anandi was sending something to Amol and writing a letter.Just to tell us that Amol though not part of their daily life is still very much a part of their life.
So it is not possible for Shivam to not to know about his father and his family