Re: why r husbands usually sweeter/nicer to their sis and mums in public but not wife
There are contradictory opinions when I researched the internet about the issue. Three examples are:
http://ashraf786.proboards.com/index.cgi?board=marriage&action=display&thread=6871
http://www.turntoislam.com/forum/showthread.php?t=46824
http://www.islamweb.net/emainpage/index.php?page=showfatwa&Option=FatwaId&Id=84311
I personally think personal contact is allowed between brother and sister if it remains within decency limits. A brother at the time of rukhsati of his sister can kiss her sister on cheek along with forehead because he can be emotional at that moment (even if he did not ever kissed her on cheeks before). The niyyat is what counts I think as suggested by ZK and the niyyat is always pure in majority of the cases.