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My brother and I show up as full siblings (2310cM shared). He shows up to cousin #1 (thru our mother) 976cM shared and cousin #2 (thru our father) 813Cm shared. However, I share with cousin #1 920cM (similar) and Cousin #2 1724cM.
This means that cousin #2 shows up as either my first cousin or half sister (through our dad). This seems unusually high for a cousin and our dads were brothers. Could a 1st cousin have that high a match if the dads were brothers?
DNA Painter suggests that your cousin #2 is almost certainly a half-sibling.
100%
- Grandparent
- Aunt / Uncle
- Half Sibling
- Niece / Nephew
- Grandchild
There are other possibilities that you can probably rule out easily such as grandparent or grandchild.
Keep in mind though that these numbers are not absolute and are based on statistics, there's no guarantee here that your cousin is actually your half-sister.
I'm somewhat reluctant to suggest this, but there's at least one scenario that fits the numbers you describe. Your match with your brother is at the lower end of the range for full siblings, the extreme high end of the range for half-siblings, but squarely in the middle of the range for 3/4 siblings. If it turned out that your paternal uncle was your biological father, the numbers all work out. That doesn't necessarily mean that's what's going on here, just that the scenario fits the match numbers.
