The 1st cousin heading on Ancestry DNA does not necessarily mean they are a first cousin, it only means that you share the amount of DNA with those people as a typical first cousin would. Ancestry does not list the full list of possible relationships, because that would take several paragraphs to do the full list in many cases.
Ancestry DNA only knows the amount of DNA you share with your matches. Except for very close relationships there are usually a number of possible relationships that could result in a similar amount of shared DNA.
I recommend using The Shared cM Project tool on DNA Painter. After entering 760 cM (or 744, the results are just about identical), it brings up a list of possible relationships. In the most probable category (86%):
- great grandparent
- great aunt/uncle
- half aunt/uncle
- first cousin
- half niece/nephew
- great niece/nephew
- great grandchild
Now it is up to you to determine the relationship based on the paper trail and/or other DNA matches. It sounds as though half aunt and half niece are the relationships you were expecting.
If there is still doubt over these relationships, consider DNA testing additional relatives such as your aunts or siblings. You would expect stronger DNA matches with these individuals.