Joint MIS account in Post office how can I claim share

I have two MIS in Post office Jointly with my mother & sister. Now in coming month these two are going to mature but my mother not interested to continue the MIS jointly with me & my sister . She planning to do reinvest this money with her other relative. I also agreed with the equal distribution of the property but she interested for the same . Please suggest me how i get my equal portion of the said money.