Father to write will for British Citizen with OCI

My father and mother have six properties (in Delhi, Haryana and Rajasthan) and I am the only son who was born and broughtup in Delhi but has been living in the UK for more than 10 years and I am now a British Citizen but holds an OCI. My parents also has mutual funds and my mother is nominee in all his accounts and mutual funds. Three of these properties were brough by my father with the remittances I have been sending to support them and also the mutual funds / FD is from the remittetted money. My mother and father has sold a plot and distributed the sale amount equally between my two sisters as gift and will be giving around 20L as loan to one of my sisters who is buying her home. For the remaining properties and assets (mutual funds/ FD etc.), my parents wants to write their will in my favour and I have some queries as follows: How to g oabout writing the will for my parents Does the will needs to be registered Is the will registered in Delhi valid for the properties in Haryana and Rajasthan or we need to do something additional for those Given my nationality is British now with OCI, do they need to write any special clauses or what factors i need to consider or do i need to notify any regulatory body/ are there any tax implications on me of my parents