Dear Mr.Dadu Nikhilendra Singh,
You have not mentioned how the property devolved and what is the proportionate share of each one. We all know that unregistered document does not have much evidentiary value more so as it is evident from your post only two out of the four siblings recorded the Memorandum of Partition which also not hold much water. The correct way to go in the situation is to ascertain how the property devolved and accordingly what is the share of each sibling. Meanwhile go to civil court get a stay to avoid any further complicity in the matter - stating that if the share of one sibling is not ascertained the sale is bad.
Best Regards
Daksh