It sounds like a big decision for you, to change where you are going to live possibly for the rest of your life.
Can you go and visit the city to try to make up your mind? When you say your property is under offer, have you found somewhere to move to in the city?
How much of the move is about being with your sister and BIL - do you also have other family, childhood friends etc in your home city, or would they be the only peoole you knew? That's quite a lot of pressure on one relationship.
You've been in your coastal town a long time, do you definitely not have a network there that you are taking for granted/forgetting about due to how you're feeling? Is it possible you move and know even fewer people than you do now?
I would try to do a taster in the city, and then also think about how you could get a fresh start in your current location. Could you get a dog, change jobs, start volunteering etc maybe with your daughter too?
Your job sounds at risk regardless, especially if you're not busy, so you could be made redundant whether you stay or move.
A bigger mortgage at 60 is definitely not a great idea. Do you have to move up the property ladder as part of the city move, not get something for the same value as yours?