tommjames:Note that "si" is third person only, so you cannot say "sentu sin libera". It would need to be "sentu vin".

Unless you are a ghost that is possessing someone and you want that person to touch themselves.

Alkanadi:I think this is a hard one because different languages use different concepts for expressing common ideas. I know this is a silly answer that doesn't really answer the question, but this is what I would say for your given situations.

You are welcome to use my car

Vi povus uzi mian auxton.

You are welcome to have some water

Vi povus havi akvon.

You're welcome," in reply to "Thank you."

Nedankinde / Estas mia plezuro / Mi felicxas helpi vin / Donu min iom da mono.

What does "You're welcome" even mean in English on a fundamental level? I don't know.

In response to "thank you", I believe the fundamental idea is "You are welcome to whatever I did that you are thanking me for." Most responses to "thank you" that I'm aware of seem to be some version of self-deprecation, telling the person that gratitude is not necessary for whatever you did.