Depends a lot on your definition. If you consider all girls in the game yours, they just don´t know it yet, then NTR is unavoidable, because there are girls in relationships or indifferent to you too. Misfits is not a harem game, you have to choose one girlfriend sooner or later.I use the search function and didn't find an answer. Is the NTR avoidable or not?
If your definition of NTR is your partner in a dedicated relationship does something "outside" without your knowledge and consent, then it is avoidable. Even the first "Betsy-Kiss" is.
Considering how the first couple chapters shaped up, the main girls you can pursue in Misfits are Clara, Amy and Betsy. Clara is the mainest girl and you shape to a certain extend how Clara will be as a girlfriend. Projected from what we know currently, she can end the game nearly chaste or a full-blown slut, with several levels of in-between.