Hey, my mother does it all the time
If you were to take their ages 'at face value' then they are very similar in age to their 'nephew', not rare but does it not make you question their ages ~ if you look at the original image you will see that whoever filled out the form originally was putting their ages under the male column, then rubbed it out and put, what appears to be a different age in the female column
OR
Another way to look at it is that the term 'niece / nephew' was a loose one at the time.
The may have been cousins and hence could be children of a sibling of Mary's.
Did Mary have a son called Patrick ? I would imagine so given that her father was called Patrick !
Where was Mary and her husband James in 1901 ?
Tara
