HR extending my notice period , based on manager request.

Hi I have been working with TCS over last 2 years, I put my papers on Jan 21st. I have also received the acknowledgment on my last working day to be April 20th in a written mail from my HR on Feb 21st. Now (April 5th) they are saying that they will extend my notice period for 2 days, as i had taken a sick leave on Jan 31st and Feb 1st as my mom was admitted in hospital, They asked me for proofs i provided them, now they are stating that sick leaves are meant for employee not for their parents, and are saying that those leaves are invalid as per policy. But i tried to request them saying, even my mental health wasn't good as my mom was admitted, i was not in a position to work so its my sickness as well. Also my question is, while i applied for leave stating that reason they dint give any objection. also when they shared my resignation confirmation and sent my last working day, that time also they dint raise any objection. is it fair to say now when i'm just 10days away from my Last working day. they are giving objection for the leave wch they have already approved! wch was 2 months back. how far is this fair? I'm going to loose my Joining bonus in my next organization if i fail to join on committed date, also my new employer is in different city, i have to relocate as well, this extension is going to create an unnecessary disturbance to my career.