Tell her what you told us. that you recieved a call from your ex
last night, and was informed that here sis is coming to singapore
and needs you to looke after for the time being (tell her why she
needs looking after too... like why your ex can't look after her),
and hope she is ok with it. assure her more than once that you and
your ex is over and that you are just doing a friend a favour.
(alternatively you can start out by telling her you are going to
help your friend look after her sis, then tell her that the friend
is your ex)
if she ask about your past, I would advice to tell her what
happened (cos, the best way to reassure her is to show her you have
nothing to hide), but if you really can't bring yourself to, then
tell her that right now you still do not really wanna think about
the past that you will tell her in the future, but just not
now.
