In general , you need to have a family nucleus to own a HDB flat.
according to what information you collect , if the 2 siblings forms the nucleus , and one of them is getting married ,then the old flat wouldn't have a family nucleus to speak of.
they might have fallen under
http://www.hdb.gov.sg/fi10/fi10201p.nsf/WP...me?OpenDocument
they could also fall under joint singles An orphan or widow/widower aged at least 21 can apply with another single citizen who is at least 35. (since none of them is above 35 , then this is probably not the case)
but this is one of the circumstances where single siblings co-share to buy a new flat?