Why notice to third parties(Me) who bought the property legally

Hi Sir, In March 2013, I bought house property from person 'P1'(she got it from her father-in-law 'F1' and her husband after their death) who has two daughters D1 & D2 , there was partition deed on D1 & D2 and my property got registered on my name from D2 legally. And I took bank loan also after legal adviser of bank. The P1 has her husband(expired long back)'s sister 'S1' she is alive now. S1 has filed case against P1 in Aug 2012 and was fighting for share in the property of her father F1. As a buyer I did not knew this and was surprised why S1 asked court to add third parties as well (like me) as defendant. The case was in pending when I purchased the property in 2013 and came For Trial in 2015. Does my sale deed and registration legal and valid or not? 1. Do I get impacted? if so, how to proceed on this? I did not knew the case and was in pending at the time of my sale deed registration. 2. Does S1 has rights to add third parties like me as defendant even though purchased from direct owner of land(who has the name in land records) legally 3. S1 name is not there in any link documents or from ancestors to till F1->P1->D2 name in land records.