How long you are actually allowed to stay will depend on the immigration officer you meet at the airport when you arrive.
There's no rule that says you cannot stay for more than 3 months. That said, if it seems like you are not a genuine visitor (Read: A stay of 5.5 months indicates you do not have any ties to return to India), the officer could limit your stay.
You may also want to keep in mind that the longer you stay when you visit this time, the higher the chances that your next visit could be limited or you might find it difficult to get another TRV in the future. BTW your sister cannot sponsor you for a trv
thanks bryanna, so should i reschedule my ticket ?