
Change your Default to:
If(ThisItem.yourColumnNameHere, "Si", "No")
Replace with the name of your column and it will give you what you want.
Before your next question "How do I get this to update to the datasource".
Change your Update property to:
yourRadioControl.Selected.Value = "Si"
That will account for both the true and false values needed to write back to your Yes/No column.
I hope this is helpful for you.