1.We were dismayed to learn that our neighbours were untidy,disagreeable, and they were uninterested to make new friends.
This sentence is incorrect because the three items joined by AND are not parallel:
-- untidy (adjective)
-- disagreeable (adjective)
-- they were uninterested (clause)
The answer provided in the book is
We were dismayed to learn that our neighbours were untidy,disagreeable, and uninterested in making new friends.
This works because the items are parallel:
-- untidy (adjective)
-- disagreeable (adjective)
-- uninterested ... (adjective)
I have 2 doubts.
1) in making is used instead of to make . Is to make a wrong usage?
This is an idiom. It should be "uninterested in making" instead of "uninterested to make."
2) If I correct the sentence as
We were dismayed to learn that our neighbours were untidy,disagreeable, and that they were uninterested to make new friends.
This is not correct, because it's still lacking parallelism:
-- untidy
-- disagreeable
-- that they were uninterested ...
If you wanted to take the last bit OUT of the parallel construction, you need to END the list with disagreeable by placing an AND before it. This (although awkward) might be correct:
We were dismayed to learn that our neighbours were untidy and disagreeable, and that they were uninterested in making new friends.
so here:
THAT our neighbors were
-- untidy
-- disagreeable
THAT they were uninterested ...
Hope that makes sense.