I entered the same parents for my mother's mother and my mother's father. I've tried replacing them one at a time but it's not working. Need assistance, please.
Yes - you can correct this mistake - and it should not be a problem - at least not that I am aware of - however obviously if both parents were siblings that could be a complicating factor in correcting Tree relationships.
I believe what you will want to do first is remove the parent-child relationships (for both incorrect parents) for the incorrect child.
Then you can make sure they are not showing as siblings, etc.
Here is a Help Center document that may help you through the process: