Property was bought in mother name out of sale proceeds for benefit of joint family 

2) sister can claim equal share in property 

3) mother can bequeath or transfer her share in property to whom so ever she pleases 

Your legal situation is different from your medical situation as the property is purchased from the proceeds of this ancestral property logically she is eligible for 50% of its share you need to work out an amicable solution to sort out this problem.

See in life of your mother ,mother is absolute owner and she can sell gift transfer the said property to whomsoever she wish , after the demise of the mother intestate without the will you and your sister shall have equal right on the property.

if mother in life gift you complete property then she has no right over it.

your mother is absolute owner she can gift entire property to anyone either to son Or daughter in her life time.

You can bring the aforesaid facts in reply in the suit filed by her to oppose the same

Share to the property will be proportionate to the contribution made by each  towards consideration of the new house. Then law of intestacy will apply. Father's share would then again be devolved upon equally to all legal heirs.

It is certainly not that your mother would be absolute owner of the whole property even it is registered with her name alone.

Hello sir,

Now the property stands in your mother name. Your mother can execute a settlement deed or sale deed in favor of any one. Being a son or daughter either you cannot claim right on the property. It is up to your mother decision.

It is also also hard to prove that the property was purchased by your father in the name of your mother. If your mother is willing to give 50% share your sister, legally you cannot restrain her. 

At present she has 1/3rd share in property after your father demise, your mother can give her 1/3rd share to you through release deed/gift deed. 

Neither you nor your sister have any rights in the proeprty which is registered on your mother's name.

In fact if your father is living, he too is not having any rights in the property owned by your mother.

None of you have any rights in the property at least not during the lifetime of your mother, this is law.

So you decide what you can do.

1. As per the information mentioned in the present query, makes it clear that there are two things to be considered.
2. First is the nature of property and Second is the situation of marriage of your marriage.
3. If the property was self acquired property of your father and he has died without living any Will then you all would have equal share including your mother and sister with you.
4. If it is ancestral then your sister might not get anything as her share was given to her by spending on her marriage as for the same House was sold and as per Inheritance law, her share gets over there only.
5. Rest, you need to go legally at each and every step to get your share and stop your mother to give anything to her out of the disputed property.

Yes your sister have right to get equal share from the property and if your mother wish she can give 100% to your sister because it is self acquired property of your mother.

So it is better to go for 50% share.