My grand mother died in 2010 and left a registered WILL, saying that the house number which is allocated to us, is rightfully be given to her elder son's son (Grandson's) and ownership be transferred over.
This WILL was presented in 2016, 6 years after her death.
Since this property was on your deceased father's name, subsequent to his death even if there was no will, the property automatically devolved on his own legal heirs consisting his wife, children and his mother.
Therefore your grandmother, if it is considered that your father died intestate, has only 1/5th share in the house property and not entire property, hence her will will can be operative only to that extent.
However as per law she cannot bequeath her share acquired out of her deceased son's property by a will, she could have transferred her share to someone during her lifetime and not after that.
Therefore the Will is not valid in the eyes of law.
You may challenge their claim on the same grounds as stated above.