Steam Families is a great new feature, very nice for sharing game libraries among family members as well.
I don’t think there’s an API for it though, and I’m not sure you need a specific integration to do what you want, at least for the Steam platform part. Have you implemented the current Steam integration? Steam - Home Assistant
When you do that, you get a sensor called something like “sensor.steam_00000000000000000” where the number is a Steam ID. You can add multiple Steam IDs of family members to track multiple accounts.
It’s not immediately visible, but one of the attributes of the sensor is called “game”, and it shows you the game being played (or “Unknown” if nothing is being played).
You can quickly test this by going to: Developer tools → Template, and in the Template editor paste the following line: {{ state_attr ("sensor.steam_00000000000000000", "game") }}
The Result on the right should show “None” until that Steam ID starts playing a game, and then it’ll show the game’s title.
You can make a Template sensor out of it so the attribute is upgraded to a sensor in its own right. You can track that sensor to see what everybody is doing, build something around that.
(I looked into this because I wanted to show the “game_image_header” as the URL of an image of the game being played, but only when a game is actually being played.)