I do know the answer to this. It’s no, NASA has no evidence of alien
life to date. We only have theories and speculation, but without
evidence there is no proof of alien life anywhere else in the universe
other than good ol’ Earth.
being said, most scientist believe life probably exists elsewhere in
the universe, but until we actually find even a fossil of a microbe of
alien life, there’s no evidence to date.
This is a great question, but unfortunately no one will be able to give a definitive answer as to the chances.
This is simply because we do not yet know if life exists anywhere other than Earth. It is still within the realm of possibility that Earth is the only place where any kind of life, what to speak sentient life, exists.
Finding such life would be an affront to many religions that contend that God created life on Earth only, and therefore finding even a fossil of a microbe on another celestial body would be the holy grail of science and have reverberating effects throughout human society. The theory of evolution would be proven.
It is believed to be highly likely that life does exist elsewhere in the universe simply by the laws of probability, but that being said, we have found no evidence to support this theory.
So now we can come back to your original question: “What are the chances of two planets from the same solar system having sentient species that reach space exploration around the same time?” Based on the fact that life has not been discovered anywhere, the chances to sentient life forms existing in the same solar system is practically nil (but not impossible).
Pretty much all of science and scientists in the world believe in extraterrestrial life, even though no evidence of extraterrestrial life has ever been found to date. Why do scientists believe in extraterrestrial life? Simply because there are billions of stars in the Milky Way galaxy and billions of galaxies in the universe. Somewhere it is highly likely that extraterrestrial life exists. It is even possible that extraterrestrial life of some form or another exists on Titan, within our own solar system!
An alien invasion by secular and militaristic extraterrestrials would solve one of the biggest problems we have here on Earth. That problem being: does life exist on other planets? Finally we would have an answer!
No alien that we know of has ever come to Earth.
As far as not seeing aliens, some aliens are microbes. Probably most are microbes. It’s hard to see microbes without a microscope.
Sneeze. That would probably kill them all off since they would not have developed evolutionary immunity to our germs. George Orwell was quite right.