If your Grandad died and left money to various people, such as grandchildren, then the solicitor looking after the will would have issued cheques to all the people when everything was sorted. The person who was executor would have provided pps numbers, addresses, and even copies of birth certificates to prove who was entitled to the money. They would have gotten a letter from the solicitor explaining what they were getting and then a cheque. It is not easy to take money entitled to another person. Perhaps your Grandad left you money but there was no money left due to him spending it before he died. The banks will not give people access to bank accounts without being told what to do by a solicitor. So figure out who the solicitor was and ask them about your money.
it sounds like your Mum or aunt has now passed away, and that their partner has collected insurance money. Again insurance companies will not give money to the first person who asks. They need proof that the partner was entitled to everything. They will have kept their own records of who they paid the money to and again copies of the will, pps numbers, etc. So if you believe you were the correct recipient of the money, go to your solicitor with all the paperwork and they will help you figure it out.
But doing all of this will cost you time and money. So unless people leave cash lying around, then the banks and insurance companies will have acted correctly and only given money to the right person. And they will have kept all the proof as they maybe asked to show why they paid out the money.
if I were you I would do nothing. The chances of money going to the wrong person are slim. However if you feel cheated and have some money to spend then go to your solicitor. Bring all the paperwork with you, copies of the will and insurance, and they will spend your money finding out the proof of what happened.