Mother and daughters' financial issue arised after father's death

My father has given me Rs. 20 lakh by cheques. On the back side of these cheques, he has clearly written and signed that "This is gifted to my daughter." I have copy of these cheques. I have used this money to construct my house. These cheques transactions were done in the presence of my father and house was also constructed in the presence of my father. This constructed house is on my name only. Recently my father is passed away. Now my mother and married-sister demand me this money. My mother and married-sister have threatened me that if I fail to give them this money, they will not give me my share in my deceased father's property. What should i do? I am a hindu married lady.