1. Probate is optional in Delhi. This being said, probating the will obliterates the possibility of a subsequent challenge thereto. Hence, probate should be applied as a matter of prudence even if the same is not mandatory.
2. Your sisters can inform the probate court that they do not have any objection to the grant of probate.
3. Stamp duty could be around 50 thousand. In Delhi it may take up to 8 months to get the probate in your case.
4. No NOC is required from the sisters in view of the fact that you and your brother have a will in your favour. NOC would have been required only if your sisters had a share in the property, but they have not been given any share in the property by your mother.
5. You are at liberty to reconstruct the house. Apply for mutation and then start the reconstruction.