Oh my gosh, I am so sorry - this is so tragic on so many levels and I'm so sad for both you and your sister. Little dogs are so vulnerable, and I've learned my own lessons about just how vulnerable they really are - and unless you're with them every day, it's hard to "just know" how to protect them sometimes.
I too have a pool, and even though we don't allow our dogs outside alone, we still installed a Skamper Ramp, it was $15 on sale:
Skamper-Ramp - SKR4