The final episode was disturbing. It actually freaked me out how ill Dan looked at the end and the revelation as to who kissed Hannah was just plain weird. Hannah has gone from being amusingly self centred to being a border line psycho as well.
Previous episodes were ok, but this one was disgusting, unfunny, ridiculous and as you said disturbing. Not to say idiotic to end the 1st season with Dan looking like that and vomiting after confess he mouth kissed his own sister.
Always Sunny is one of my faves but never really made the link before with Siblings until you mentioned it. There are so many similarities between the siblings in both shows I wouldn't be surprised now if Siblings was based on Sunny.
The final episode was easily the best as the other episodes were formulaic.