How lovely, I'm a fan of middle names being after family members, and Lainey is very cool in my opinion!
Great choice.
You might get people comment about shortenings of the name, Theo for example is quite popular for boys as a name, or people might think she'll get called Dora. Something I have noticed with the current tween/teenage population is they tend to say names in full these days, and don't automatically go to a short version, so if you prefer her to be called Theodora in full, I think that will be a lot easier and more normal than others often say on here. Kids these days seem much more open to a wide variety of names from various cultures, themes and styles. Of course if you like Theo or Dora kids will go along with that too.