Am I subject to any disadvantages when sharing Steam games?

Like most people on Steam I have stockpiled a large library of games over the years. I recently found out that there is a Steam Family Sharing feature, which seems a great way of letting others make use of my games.

Specifically, I have a family member in the same household with their own PC and another family member in another city with their own PC. I would like to share my games without limiting my own use of my games.

• Do I need to login/maintain an account on these other machines?

• If someone is playing a shared game do I get locked out of my game library until they are done?

• If they do something stupid like obtain a VAC ban am I subject to the same punishment?

• Are there any (other?) disadvantages to myself if I share my games?

Do I need to login/maintain an account on these other machines?

No, just once to activate Steam Guard on their machine

If someone is playing a shared game do I get locked out of my game library until they are done?

No, they will be kicked out of their game to allow you to play. Per Steam Family Sharing FAQ:

When I authorize a device to lend my library to others, do I limit my own ability to access and play my games?

As the account holder, you may always access and play your games at any time. If you decide to start playing when another user is already playing one of your games, he/she will be given a few minutes to either purchase the game or quit playing.

Clarifying for @WernerCD - any game activated on Steam will kick the person with whom the library is shared.

If they do something stupid like obtain a VAC ban am I subject to the same punishment?

Yes, your account might get banned for any cheating/fraud conducted by the one you share your library to. Per Steam Family Sharing FAQ:

Will I be punished for any cheating or fraud conducted by other users while playing my games?

Your Family Library Sharing privileges may be revoked and your account may also be VAC banned if your library is used by others to conduct cheating or fraud. Additionally, VAC-banned games cannot be shared. We recommend you only authorize familiar computers you know to be secure. And as always, never give your password to anyone.

Are there any (other?) disadvantages to myself if I share my games?

Not really, only disadvantages are on the other side (limited availability when the library owner is playing, no offline mode to play shared games, no trading cards gathering, etc.)

Steam Family Sharing FAQ can be found here.

Asunez already posted a great answer to this question so I'll just add a few things.

You will need to log in each of the machines your relatives play on. If your relative owns multiple computers, you will have to log in on each computer your relative wants to play your games on. If you authorize only 1 computer, your relative will be able to play on that device but not any others.

You are never locked out of your own games.

Even if you play a free-to-play game from your steam library, other's will not be able to play any game from your library.

If the game also has it's own launcher that doesn't need Steam I think others can make use of your library while you are playing, but this needs confirmation.

You can not play multiplayer with your relative if you own the game, share it but your relative does not own the game.

Someone who plays a game shared by someone else will be able to earn achievements but will not earn Steam trading cards (unless they own the game themselves, of course).

I haven't encountered any disadvantages other than having to log-in and authorizing my relatives computer.