This is why I believe companies should stop using MMN as security
. Your siblings - including half siblings who you share the same mother have the same MMN
. If your mother has sisters and they have children too - they also share the same MMN
. Mothers who split up with their child(ren)'s father or decide they have nothing to with the child(ren)'s father(s) usually have the same surname as the mum if the mum wants to revert back to her maiden name (if got married) and kids get surname to match their mum's - their MMN is their surname!
. Anyone can go on free websites or have a free trial to find birth registers and if you know the quarter their births are registered - can find MMN with ease.
For companies that insist on a MMN - I use a family surname which is none of the MMNs of my parents or grandparents.