The Sand Snake girls are introduced in late 5th season of Game of Thrones. The eyes of Bronn started burning when he looked at one of them.
What is the power of the Sand Snake girls?
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I think you are referring to their scene together in the prison.
This makes Bronn's heart rate increase, his nose bleed, his vision blur and he eventually collapses.
Tyene then gives him the antidote, but only after he tells her she is the most beautiful woman in the world.
So their power is simply that they are well trained in combat and, like Oberyn, often coat their weapons in poison.