My question is: How can two sisters be maternal aunts through fosterage?

This is mostly possible and has several scenarios:

1. Two sisters by lineage have a sister who breastfeeds a child. The two sisters then become maternal aunts (khalat) to the child through breastfeeding, because they are the sisters of his mother through breastfeeding.

2. Two sisters by breastfeeding, and the woman who breastfed them has a daughter who breastfeeds a child. The two sisters then become maternal aunts (khalat) to the child through breastfeeding, because they are the sisters of his mother through breastfeeding.

And if the intention is for each sister to be a maternal aunt (khala) to the other through breastfeeding, this is also possible. The scenario for this is that both sisters are breastfed by their grandmother (their mother's mother, whether by lineage or breastfeeding). Thus, each of them becomes a maternal aunt (khala) to the other through breastfeeding.