its not a custom or anything. gujaratis dont call eachother bhai or ben. its just that if another person is not in any sort of relation like mama, chacha, chachi, etc. , out of respect just a bhai or ben is added, just like u add 'ji' in hindi. of course u also call ur own brother and sis bhai and ben respectively.
and as for everyone calling her jasuben, maybe her devrani pankorba or husband used to call her that. and kids just repeat what they hear from elders. so since childhood, they would be calling her jasuben. i remember i used to call my chachi as bhabhi, as my parents did so.