Issue in the mother deed

I purchased a land in 2011 from person "A". In 2015 I had my legal checked and applied for a loan for construction which got approved, also part of the loan amount released & 30% of the construction is also happened. Now someone "B" stating that his grandfather sold the property to "A" but they failed to take signature from all the son / daughter, as per the mother deed "B"s Grandfather sold the property to "A" in 1996 and "A" sold it to me in 2011. "B" is now asking for money from "A" and myself for continue with the construction. They are threatning me that they will get a stay order if we do not pay the money. I am a bona fide purchaser, I never knew such issues nor "A" who sold it to me. It is very hard to believe that someone file a case after 19years specially when we started the construction. Is the concern raised is valid? What should I do? Do I need to pay the money to "B" :-( Can I continue with the construction Do they get stay order in such scenario Please help me to clarify these question. Note that this is the B Katha certified property and it was a land when "B" sold it to "A", but "A" converted to site and sold it to two people. Some people said that law changes depending on the caste, hence providing the details of the caste..."B" is SC caste, "A" is General Merit & I am also in general merit category.