To hide your activity on Steam Chat, click "Friends" and set your status to "Offline" or "Invisible." To hide games from your Steam library, right-click the game, then select Manage > Mark as Private.

If you also want to hide your girlfriend's chat from your siblings and friends, then there is a way with the help of which your private chat will disappear from the chat list without deleting it.

Tap on the down arrow icon at the top of the screen to hide or archive the chat. The chat will be moved to the archive folder, which you can access by scrolling to the bottom of your chat list and ...