1st cousin overlaps, so it seems to me it’s possible that either could be the cause of the same level of commonality if it’s in that overlapping range. Does that make sense?

A half sibling has 25% in common with you *on average*.

A First cousin has 12.5% in common with you *on average*.

But we’re taking about one individual, not a large sample that will be necessarily close to the mean.

So look at the ranges:

Half sibling will be in the range of 17%-34% in common with you.

1st cousin will be in the range of 4%-23% in common with you.

So if this individual hypothetically had 22% in common with you, she would be much closer to the average for a “half-sibling” and would probably be listed that way on your report (I assume). But that’s still within the possible range for a 1st cousin too, so it wouldn’t necessarily be conclusive. That’s your plausible deniability for your dad and aunt.

If she had 24% or more in common with you, it would be very unlikely that she’s a 1st cousin, and much more likely a half-sibling (unless the respective non-sibling parents are also somehow related).

Do you know the actual % variable DNA you share?