You need to trace/track ownership from Grandfather to date. The fact that change of ownership on everyones death was not dealt with by anyone does not make the tax/title issues go away.
Did GF make a will? If no, who was entitled to the estate? Your father, your mother? Neither of them appear to have made a will so trace ownership on intestacy rules. Its likely that the property can properly vest in all 6(?) offspring and 2 are disclaiming any interest. After that, 2 want sites, 1 will take 1/2 acre and 1 takes everything else. So that last person is taking more than everyone else. It does not matter that they built houses - did it never occur to anyone to sort the mess out when they were doing that? They are living in houses on land registered in someone elses name. That is where the potential tax issues arise.
Why not ask the solicitor to write to you breaking the transaction down into stages so that everyone is clear?
mf