Misuse Of Power of attorney

I (say Mr.A) have purchased a plot in 1998 from Mr. B. Mr.B was having power of attorney from his organization to sell that plot. Then in Feb, and 3rd March 1999 Mr. D (Mr. D also claim to have power of attorney from the organisation) sold 74% of that plot to Mr.E. by two registered deeds. Then In 6 March Mr.E sold 50% of his 74% of plot area to Mr. F. further Mr.E and Mr. F had done Mutation in their Names with Nagar Nigam and have physical possession of plot Now In 2014 have field a civil suite against Mr. D, Mr. E & Mr. F. (Because of Stamp duty issue my case is not time bared) Now in 2014 a lawyer of purchaser Mr.G approached me and told me that My sister has made an agreement to sell that plot for Rs.16 Lacks. Now I came to know that (I have given Power of Attorney to my sister in 1992.) My sister without my consent made an agreement to sell my plot in 2002 for sum Rs.16 Lacks with Mr G . She had taken all amount. and deposited part payment 4 Lacks by chq issued by purchaser Mr.G and 2 lacks by her chq. in 2006 in my account, and asked for registration. My question is 1) Whether My sister and MR. G have any legal right to enter an agreement when already 74% of plot has been sold and their Registrations are not cancelled? 2) As she has not made the agreement with my consent, I want to break it. How I can break? Mr.A