You would not find any buyer for your undivided share in flat 

you can file suit for partition for division of flat 

if flat cannot be divided court would direct flat to be sold 

If it is a vacant land then you can sell the undivided share in the property, but since this is a jointly owned  apartment there may not be any possibility to sell your undivided share in the property.

Your unwilling sister would obtain a stay order against you from court of law on this  decision.

Even the court may pass an order to bring the property to sale if the property is indivisible. 

In any case the buyer may not come forward to buy an undivided share in the apartment set up property.

Since your sister is a joint owner of the proeprty, it becomes your duty to inform her about our proposal to sell your undivided share in the property.

You should inform your sister about sale of undivided share in property 

Yes you can sell only your share but possession will be an issue in the same in future

You can sell your undivided share 

Before that you can offer your sister to buy your share

If she refuses you can proceed to sell to a third party

The buyer can then buy your sister's share

If she does not sell then both will have to hold the flat as tenants in common 

Dear Sir/Madam,

You are suggested that it is practically not possible to sell that property to any one and hence you are suggested to go for the partition of the said property first and then for the sale. 

co-owner can sell or transfer his portion only when he has exclusive rights to that portion of the property. 

before selling you need to inform your sister. .