Heirs to their mothers
Heiresses, so far, have been to their fathers. This was because the fathers had the property.
But some wives who were themselves heiresses had their marriage contracts drawn up so that their heirs were the heirs of that marriage. Their husband could have been married before and had many sons, but these sons were not heirs to the property of the new marriage. The new marriage might only have had daughters. These daughters would not have been heirs to their father, that elevated role was for their half-brothers. The daughters were then heiresses of their mother.
In all other respects, things then happened in the same way as for other heiresses.