For instance, if I am having a chat with my sister about some decision I made, she'll go "Oh I would have done this" or "IDK - is that true? I've never heard of that. That must not exist" (she's a bit of a spaz and will change her mind but be sure of whatever it is that she's thinking)
Then a sec later she'll be like "Oh whatever you did you did, no need to regret, or yah im sure its fine" but in the meantime I'm totally questioning myself, researching online, trying to make sure that I made the best decision and totally unhappy again. I try to rationalize but ugh it takes all this time because I want to make sure that whatever decision I made was alright

Is this a vent?

You are two separate people. She can pick her stuff, you can pick yours. I'm puzzled.

A.

Is this something you feel only in relation to your sister or in other situations, too? If it's just your sister, then I think you guys should work on it. I'm an only child, but I see with my dh and his brother that some of their modes of relating to each other go back so far that nobody knows how they got started and some of them are so tied up with longstanding family mythology that their nearly impossible to untangle.

Is your sister the older one, or the "smarter" one (I mean as far as the family mythology goes, iykwim), or the one who's always right?

I know what you mean, doesn't matter how sure I am, if someone else says something negative or opposing the decision I've made or I find myself second guessing and researching everything to make sure I made the best/right decision. Try to have faith that you made the best decision for you in the first place, tho I know that's easier said than done.