1) ideal solution would be to apply for eltters of adminstration from court as father in law died intestate
2) enclose FIL death certifcate
3) the other two legal heirs can execute consent affidavit in favour of your mother . it should not take more than 6 months
4) for issue of legal heir certifcate you have to file petition before court . enclose father in law death certifcate , name of legal heirs .
5) legal heir certifcate is not for transfer of proeprty in name of the legal heir . for that you need letters of adminstration from court