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Note: There are higher resolution images but they are part of monthly pack with other images so I don't put them here.

That is Clearly Euryale.

Base +7, Udry +6, constellar +6, Ridr +6

It also says "younger" sister, so it's pretty clearly Euryale and not Stheno (though technically she's the middle sister.)

Base +9, constellar +6, Ridr +6

This is Euryale not Stheno though. The way you distinguish between Euryale and Stheno is pretty simple.

Euryale wears a silver bracelet because she's a 3 stars servant.
Stheno wears a gold bracelet because she's a 4 stars servant.

You can also tell them apart base on their hair band

Base +6, constellar +6, Ridr +6