The housekeeper knowing her well only means the sister visited the home which doesn't seem odd for a fiancee to do.
I think the Sister probably tried to get the ML to "warm" to her and made alot of effort in trying to "romance" him but he didn't reciprocate the way she wanted. In frustration, she ran off with another man or maybe thought the ML might take notice of her if he "lost" her. Instead, the ML didn't seem to care that much and moved on to her sister as a replacement to fill the duty.
Notice, the sister looks a little sad when the FL talks about how well their relationship is going and that the ML even confessed to her.
The sister is used to being pampered by those around her, especially her parents, and probably didn't know how to deal with the ML's indifference so instead of being direct she threw a hissy fit by running away with another man.
That she only *might marry only AFTER the ML and FL are married.
The sister seems very immature and emotionally spoiled.
So were ML and sis ex-lovers?? what's their past that even the housekeeper knows her well?? if the sis did have a past with ML why can't she be affront with her?? the sister acting suspicious is what I hate the most!!