if father was suffering from dementia and you have evidence to back your claim father was not mentally for to execute the will 

2) litigation is long drawn process and suit may take 10 years to be disposed of 

3) on mother demise you have 40 per cent share in property 

4) you are at liberty to sell your share in property however your siblings may seek orders  to set aside sale on grounds that in case of sake you have only 40 per cent share 

5) C can execute will bequeathing his share in property during his lifetime 

You can do the point no 1 accepting and later after demise of mother go forward but once after acceptance you can’t later challenge the same will

C may not be married but on demise of your mother he will have share in property as per will.

on his demise intestate property would devolve on his siblings 

It is necessary to peruse will to advise further 

The Will states that your mother is vested with life interest.

This implies your mother has a life interest (the right to live there or enjoy the property) until her passing. No division or sale can happen without violating the will until then.

Even though your father specified square footage and floors, a house is structurally a single entity. Unless the property is officially partitioned into separate legal titles with the local municipal authority (which requires independent utility connections, structural safety approvals, and separate tax assessments), you cannot easily sell "1,100 sq. ft. of a ground floor" to an outside buyer. Most buyers and banks will not touch a fractured property without a clear, independent title deed and it can be considered to be defective title.

The joint conclusion clause acts as a safety valve written into the will. It says if you jointly agree that physical distribution is impossible, you sell the whole house. If you try to sell your part independently, siblings B and C could legally block the sale by arguing that the property cannot be physically or legally split, thereby forcing the "sell the whole house and split 40/30/30" clause into effect.

If you accept the will, you are bound by its terms, including the risk of being forced into that 40/30/30 split if a joint physical division fails.

If you choose Option 1 (Contest the Will) and the legal battle outlives sibling C, what happens to C's share depends entirely on when C passes away relative to the final court judgment.

Since C is unmarried and your father has passed, C's primary legal heir under most succession laws (like Hindu Succession Act or standard civil laws) would be your mother.

If your mother is also no longer alive at that point, C’s share would be divided equally among the surviving siblings (you and B). It does not automatically "go back to the estate" to benefit you exclusively; it is treated as C's personal asset passing to C's heirs.

If the court rules that the will is invalid (due to your father's dementia), the property is treated as if your father died intestate (without a will). Then the intestate succession law will prevail.

You mentioned siblings B and C are open to taking proceeds in cash, which you oppose. In real estate transactions, demanding "cash" (unaccounted or under-the-table money) is often an attempt to evade capital gains tax or stamp duty.

Engaging in undocumented cash transactions carries severe financial and legal penalties. If the property is sold as a whole, insist that all transactions go through official banking channels (check, bank transfer) to protect yourself from legal liability.

In legal terms, a will does not need to specify what happens to a beneficiary's heirs. If the will is accepted and C survives your father, C’s share becomes C’s personal property. What happens to that property if C passes away is governed by Succession Law, not your father’s will.

If a lawsuit to contest the will takes 10 years (which is, unfortunately, a very realistic timeline for property litigation in India), the question of who physically and legally holds the property during that decade depends entirely on who is currently in possession and whether an Executor was named.

The law generally prioritizes maintaining the status quo while a court decides the validity of the will. 

As long as your mother is alive, she holds the primary legal right to possess, reside in, and enjoy the property.

Even if you file a lawsuit to contest the will, the court will not throw your mother out. She holds the right to occupy the house or collect its rents to maintain herself during her lifetime.

If an Executor is named: Legally, the property "vests" in the Executor during the litigation period. They hold the legal title as a trustee until the court determines if the will is valid or invalid. The executor could be your mother, you, or even sibling C.