Yuri no Yakata Monogatari ~Himegimi no Kaben to Maid no Tsubomi~

An old mansion stands quietly deep in the woods.
While taking a trip Fumio becomes lost, and solicits help from this mansion.
Appearing before Fumio is the beautiful head of the household, Reina, and her servant Batra.
Fumio is welcomed in curteously, but that night she happens to witness Reina and Batra together in bed.

Seeing two women make love for the first time sparks a fiery desire in Fumio.
Fumio becomes captivated with Reina's enchanting beauty and completely entrusts her body to her.

And then from that day things started at the mansion was more days.
As if guided by a mysterious fate, Fumio goes on to being swallowed up by the two women's secret whirpool.

[From English DLSite]