I don't think it's fair to say that the fact that Luke addresses them with the titles of Uncle and Aunt implies they have a sour relationship.
A relationship doesn't have to be actively bad/"sour" to be relatively loveless. I agree with what people have said about the emotional logic framework of movies, but if we stick to the in-universe text, I don't see much evidence that they were all that close. Doesn't mean they weren't
... but I think there's room for differing interpretations.
two people who have raised him from birth
Again, while probably
the case, this is never explicitly stated in ANH
, and is therefore speculation.