What you've said may be as good as a description as there is, otherwise, the gesture could be interpreted in more than one way. Colin mentions pumping an open palm down toward the ground, as if one is dribbling an imaginary basketball, which I'd usually interpret as "Keep it down!" or "Turn down the volume!" but could also mean "Be quiet!" in small room. Yet another gesture is to put one's hand horizontally across the throat, palm down, and then pull it in the direction of elbow with a sharp yanking motion. This is often translated as "Cut!" but is often used as a sign to mean "Cut! Quiet! Shut up! Stop talking now!" when someone is about to spill the beans, like your uncle was about to do.
The first term that came to my mind was shh sign, but that won't be any less subject to interpretation than hush sign, shush sign, or be-quiet sign.
I've seen the gesture you descibed used in an attempt to halt a conversation, or to exhort the participants to speak more quietly, but not necessarily shut up.
All that said, instead of scrounging for a universally-recognized name, I might opt for some more vivid language that gets the message across unambiguously – something like:
During conversation with my uncle, he was telling me their future
plans to sell house when my Aunt stopped him by hastily putting her
index finger to her lips and gazing at him with the intense glare of an angry