Mother was Hindu at birth.

The first child (20yrs old now) was born to her with her first marriage, which did not last.

The 2nd marriage was done with a Muslim, and thus converted to a muslim, due to which the mother’s family disowned her.

Now she has 2 children. But the elder child is not able to do anything knowing all this situation.

The mother already know the azab and ill effects in the afterlife. But chooses to continue

What is the child suppose to do?

Tell her step-father, or let the things be the way they are?

Better you contact local alim and involve your elders.