@Juliani,like someone has mentioned,that was a real test man and you passed with flying colours when most of us would have gotten an F for Funda (no offense Fundaah) so don't be so hard on yourself.
It looks like you're really desperate to make things right with the sis (I hope for the right reasons) and I've not seen one single reply asking you to just sit the girl down and talk to her. Guys,with the hindsight of experience,we all know what goes on in the mind of a chic who's in 3rd year colle,she was probably attracted to the whole 'older,probably responsible,can offer security' sorta guy that you epitomize in her eyes. Coupled with the sibling 'let me outdo her' mentality that's more the norm than exception,it's no wonder that the situation played out the way it did.
Actually,ladies in the house,that's how we end up sleeping with the plain mboch doing all those things as you watch a soap opera (of all things) that probably has the same theme as what's going on in your household,then you wonder,'oh where did I go wrong?' I wish hata angekuwa a no-nonsense career woman who neglected wifely duties coz she wa making the cheddar,but watching soaps (hata si Dstv?!),damn!
Anyway,I digress. Just sit the chic down and explain everything as it is (don't take the opportunity to impress her with a mercy shag,that will ruin things!),and explain that you really apreciated every thing she's done,and that it's not that she's not fly (hata kama ni kiatu) lakini her sis is yo wife and she should put herself in her sista's shoes.
If that doesn't work then throw the b**** out and hope the saga doesn't come back to bite you in the ass! Let us know what happened man. Good luck...
smano
BEER IS LIVING PROOF THAT GOD LOVES US AND WANTS US TO BE HAPPY!