If you are a Singapore citizen who owns two properties (albeit partial interest in one property), the acquisition of your sibling's share would be counted as your third property and subject to the prevailing ABSD rate.

However, as you are a Singapore citizen who owns two properties and acquiring additional interest in one of the two properties which you already have a partial interest in, partial ABSD remission may be applicable under Stamp Duties (Transfer of Interest in Property which Buyer has Interest) (Remission of ABSD) Rules.

Please note that BSD is still payable on the acquisition of your sibling's share in the property.

You may refer to Additional Buyer's Stamp Duty (ABSD) for more information on ABSD.