DRX, erm...... I don't know, I think this may be a delicate
situation for your gf. You've told us before that she's still kinda
insecure because she was the 1 who expressed her feelings to you.
Now, with your ex's sister coming into the picture, perhaps your
ex's mother feels you're trustworthy enough to take care of her
other daughter...
Your best bet is to be honest with your gf, & tell her that
whilst you loved your ex, she's now the 1 who's in your heart.
However, since your ex's mother has asked you for a favour, you'll
do your best to help. However, do the minimum for the sister,
finding her a place to work & a job. Explain to her mum why
you're unable to "take further care" other than the duty under
which you're bound.
This is the best solution I can think of for now 
FeowFeow
