Tiamat goes likely back to that legend the skalds told of the vampire creation myth by another race that Inanna likely is a part of but possibly there under another name, which well may be Tiamat or something else again. VG has quite a few of the attributes of Tiamat, though there is of course also a link between Tiamat and water to take that another way. Well who knows maybe both sisters you propose are a part of Tiamat or were Tiamat once or within game daughters/childe of Tiamat? That is the fun with a story like this, devs can come up with their own twists based on mythology but do not have to stay fully true to it
Just for sure that creation myth those skalds told was not being told for nothing and either Inanna and/or Tiamat will be part of that creator race and the antagonist will likely be as well if Inanna's sister or a member of that race vampires were created to help fight I suspect and that could well be Tiamat as well in the end, she is kind of linked to a war with the other gods after all. Hints are there, where the devs want to take it in the end though is more of a question at the moment.