Will issues

Hi I am a senior citizen from rohtak, haryana. My father passed away about 3 years back. We are 3 bothers and a sister. My father gave away most of his property to my sister through a will which he supposedly made while he was on death bed. This will was shown to us by our sister and now we have contested this will. I want to ask.... Is there any law or a recent judgement that the Will has to be made in the presence of all legal heirs involved? Can the person making a will on a death bed make a will without informing about it to his sons? Someone please update, also please confirm if there are any recent supreme court judgements on the above stated facts.