Really?? I think they both have doubts.
http://www.mangago.me/read-manga/magi/bt/v20/c298/pg-15/
Here Masrur is warning Alibaba not to follow Sin too much because he's only a human.
Another one,
http://www.mangago.me/read-manga/magi/bt/mtd_chapter-304/pg-6/
Jafar doesn't seem to quite agree with Sin and Sin wants to talk in private with Hakuei. Like, if Jafar doesn't think that unnatural he must be really brainwashed!
Why would a princess of Kou try to crush her own brothers and sisters? And why would he be more intimate and confiding everything with her than with Jafar?
Masrur also says to Alibaba, "there's someone else beside Sin now" "It's fine if you don't know." I think he's hinting that he doesn't agree with what's going on with Sin and Hakuei and it's better not to know about it.
OMG!!! So Sinbad's really changed.( ´•ω•` )
But this explains everything. It's kind of interesting to think neither Aladdin nor Yunan really trusted Sin, and how right they were about it.
I'm so glad Hakuryuu's helping out!