system
A chat on subreddit /r/NoStupidQuestions
human
1. Gravity. Earth's gravity tugs on other bodies in our solar system. This sister planet would too.

2. It would be an unstable orbit. One of the two planets would not be able to remain. Technically neither would be planets, either, if this did somehow occur.

3. Not all telescopes are on Earth.
gpt
What telescope is furthest from the earth?  I guess cameras on vehicles sent to Mars or other planets probably count and might be able to spot such a "hider" planet.