Hi
First and foremost your builder is the person who should indemnify you of costs in this law suit .
In the event of you receiving summons from court, it is better to appear in the case through an advocate
In the event of you not receiving notice, you can check through a lawyer to see whether you are arrayed in the suit as a defendant and attend proceedings if arrayed.
Technically and legally you will not be impacted in any which way by this suit, except that there is mental tension for you till the suit is pending in courts.
It is also recommended that all or majority of the flat owners decide on a single law firm who can collectively represent all of you and get all of you discharged from the suit as
a) Legal costs can be shared by all of the flat owners
b) Multiple lawyers only unnecessarily delay the process. So if all of you(even if 2,3 or 10 of flat owners) collectively appoint a law firm, then the law firm can represent all of you and seek early discharge for flat owners from the suit.
c) You should ask the courts to discharge you from the suit on the grounds that you had purchased from the builder and were not in any which way connected to the dispute between the grandson and grand father and let builder fight the case to absolve himself as he had purchased the property from the grand father.
d) To provide you with additional comfort, if the father of the grand son is alive, the grand son will not succeed to grand father's property as long as father is alive by virtue of section 8 of hindu succession act. Grand son is not a class I heir to grand father's property when father is alive. Commr. of Wealth Tax. Kanpur v. Chander Sen, (1986) 3 SCC 567: (AIR 1986 SC 1753),
Hope this information is useful