I thought I understood this and that you have to base gender and number on the subject (first noun) as it was done here in the examples. So I would say, and have in fact been saying "Elle est plus grande que son frère" and "Il est plus grand que sa sœur".
Now I have a workbook that told me to write:
"Un village est plus petitE qu'une ville"---> Using the gender of ville, the second noun in the comparison, rather than the gender of village. That's wrong, right?
It also told me to write "Paris est plus grandE que Bordeaux". The only explanation for this I have is that they are "La ville de Paris/Bordeaux" here and hence feminine. Does that sound right?
I think it's time to recycle my workbook...