What is the legality of an unregistered will

My mother was running ill and was loosing hope day by day. During her illness she made her hand written WILL (unregistered) in presence of two witnesses. Unfortunately she could not survive to get her WILL registered. After this incident (1 yr later) my father made a registered WILL and in my father’s WILL there are clear details of my mother’s WILL with date of execution and other details. BOTH THE WILLS TALKS ONLY ABOUT SELF-ACQUIRED PROPERTY. My parents did not get any ancestral assets. Entire share of SELF-ACQUIRED Property (in Pune) in both parent's Wills is given to son instead of equal division between son and daughters. My questions are 1.Now does my mother’s WILL act like as a registered WILL? 2.Mine is a Hindu Family and I am the only son. Can my sisters (Two Numbers) challenge my Mother’s WILL, since it is unregistered? 3.Can I legally press my sisters to abide by my Mother’s WILL? 4.What further need to be done from my side that I should not get into any legal issue?