Yes, that is bad practice, ummmm. Good solicitors NOT to have, in fact. But yes, if they were already representing the other side when Mum first went to them then they did the right thing by not taking Mum as a client. If however the cousin became a client after Mum hired them, that sounds like malpractice.
If these other incidents are taking place without any witnesses, I guess there's nothing much the police can do. But I still think the original theft is a criminal offence. Try the CAB, as someone suggested, they may be able to suggest possible remedies.