1. the property is the self acquired property of your parents
2. they have made mutual Wills [i assume] by which the property is bequeathed to you and your brother jointly
3. your father is no more. So his share in the building will devolve on his legatees [i.e. his 2 sons] as per his Will
4. mom is still alive, so her share in the building will be continued to be owned by her
5. since the property is in Mumbai, the Will has to be compulsorily probated
6. thus as for now and subject to the father's Will being probated, the building property is owned by your mother, yourself and your brother jointly as co-owners
7. your son from first wife cannot claim any right over this property so long as you are alive
8. the son's right will come into play only after your demise and provided you die without leaving a Will
9. if you die intestate, then your son from first wife being a class 1 legal heir will be entitled to your share in the building alongwith your other class 1 legal heirs who are your mother, widow and other children
10. however if you die testate, i.e. by leaving a Will, then the devolution of your share in the building will be by testate succession i.e your share will go the legatee named in your Will in whose favour you may have made a bequest
11. the son from your first wife as of now cannot claim any share in the property which you have inherited from your father under his Will.
12. Even if your father's Will is set aside for a moment, on demise your father, his share in the property will devolve on his class 1 legal heirs, which does not include a grandson!
13. hope this clears
14. ancestral property is one which is inherited by a male Hindu from his father, grandfather or great grandfather
15. this property i assume must have been bought by your father out of his own funds and therefore it is his self acquired property
16. if your son wishes to claim in this property then he will have to first prove that this property is a HUF/ancestral property in which he too is a coparcenor. He will have to file a partition suit
17. to repel any such action by your son, you can produce the sale deed by which this property was purchased by your father out of his own sources of income.