Sasuke would probably be indifferent at first. He is more focused on his own goals of getting stronger. But if Tora somehow gets involved in a situation that affects Sasuke's plans or if Tora shows some useful skills, he might start to respect Naruto's decision to keep Tora. For example, if Tora helps them track an enemy ninja.
I'm not sure if there specifically exists a 'Naruto has a Tora lemon fanfiction'. Fanfiction covers a vast range of ideas, but 'lemon' often refers to adult - themed content which is not appropriate in many contexts. There are lots of normal Naruto fanfictions out there, though.
One prevalent theme is the exploration of Tora's character through his relationship with the OC. It could be about how the OC helps Tora grow or change in some way. Also, themes of friendship and loyalty are quite common. The OC and Tora might face challenges that test their bond, and how they overcome those challenges is often part of the story.
One popular 'poppi x tora fanfic' could be 'The Adventure of Poppi and Tora' which focuses on their friendship. It has great character development and an engaging plot.
A non - lemon Naruto Tora fanfiction could be about Naruto befriending Tora. Maybe Naruto is on a mission to find Tora who has run away again, and during the search, he discovers some interesting things about Tora's habits or the places it likes to hide. It could also involve Naruto trying to train Tora to be a little ninja cat in a humorous way.
It could be that Tora is attracted to the unique characters in Naruto fanfiction. There are so many different personalities and powers that might be appealing to a cat's curious nature.
One possible plot could be that Naruto is assigned the task of taking care of Tora long - term. He tries to train Tora to be a ninja cat, teaching it basic stealth and combat skills. Along the way, they form an unbreakable bond and go on mini - adventures in the village, getting into all sorts of comical situations like chasing mice that are actually small summons gone astray.