This isn’t an issue for most of us I am sure but there are likely many new players who would love to know. If we could tell our friends that the new map can work entirely on its own I am sure many more people would play. Just trying to get the story straight for free advertisement moving forward.
Yes, the base game is required to play the IoS. Unfortunately the free weekend on Steam is over.
Yes, you will need to buy the base game if you do not own it yet before you can play the Isle of Siptah DLC (the order in which you purchase them does not matter).
All DLCs require the base game to function; they receive updates from it and use its assets. The map filter (selection) feature is integrated with the base game as well.
It’s on sale till September 21 if I’m not mistaken - a bundle of the base game and Siptah DLC.