Relatives inappropriately using ancestral property without consent

My grandfather (passed out) gave 20000 to my father by applying loan on bank and my father has returned 25000 to my grandpa.our grandpa has 1acer and had three sons and my father is last of them.my fathers big bros wife(bada ammi)has claimed our grandpas land by(she need loan so bank manager said all the property should be ones name then loan will granted) my father beloved her and his big brother and signed in agreement or papers. Now she changed land on her name.and she saying ur father has take loan 20000 and now its interest has been 90000 I paid it and take a new loan if u paid interest then I will hand over the land but my father has paid the money with interest to my grandpa.what should we do.can I have legally rights on my grandpas property and how loan issue be solved.plz give answer.