After a certain time passes, these things cannot be opened up and unscrambled unless there has been fraud (Statute of Limitations.)
You should consult an advocate and they will want to know whether grandfather was married to the new lady, and when he passed away.
The first part of the estate goes to the spouse, and then after that it gets more complicated.
It is all set out in the Administration of Estates Act and Orders made under it, which you could read in the Tynwald Library .