My mother is the first wife but my father added second wife name in SR(My father passed away 6 years

My mother is the first wife but my father added second wife name in SR(My father passed away 6 years back). - My mother marriage happened in the year 1975 - My mother files maintenance case in 2009 and the case was ex-party for my father since he did not attend any hearing. - Court ordered 5000 maintenance per month. - Then my mother filed amount recovery suit and my father gave 1 DD in court which was closed in 2011. - After that my father used to deposit 5000 rs everymonth to my mothers account which we are not sure if it is the cash deposit or account transfer. -My mother is having voter id from my fathers place which is issued from 1995 and my fathers name is mentioned in it. Now, after his death we came to know his SR has his second wife name. -Case is in evidence phase where we need to prove my mother is the first wife. - What other evidences can help us and how to get them? - My advocate is saying get voter list from 1975 which seems impossible for me (Is there a way to get voter llist from 1975). - My mothers marriage was not registered. Please help on how to deal with this case.