Property Transfer rules for Property bought by my father

My Father bought Flats and shops at Virar. He named it on his siblings and his father name out of affection. Now my father has passed away this year. All his relatives acknowledge the property as rightly ours, but do not want to transfer it to us legally. Now we want a way to ensure that after the death of respective relatives whose name is their on our property, we get the property and not their successors. They say they will make a WILL on our name but the fact is that itcan be challeged in court. Thus, what is the best alternative: Become co-owners in all property or what else can ??