What your sister describes is not uncommon. Usually it is a trick of
the mind. After years as a caregiver, when a parent dies, we can get
a sense of the departed in familiar places. I suppose the force of
habit makes the mind see or hear the deceased. Some report smells
like familiar perfumes. Could it be more? As a priest, I would have
to say yes, but it would be less likely. It could be a loved one
asking for prayers of final deliverance (from purgation). There is
also another possibility which I hesitate to mention. A classmate in
seminary thought he saw his dead mother in a mirror. Well, to make a
long story short, it was not his mother.
If you would like to know more about that story, here is the link on
my blog. But please be warned, it is a bit unsettling and probably
not the thing to read at night. I do not think it should be shared
with your sister either:
http://fatherjoe.wordpress.com/stories/face-in-the-mirror/
I would simply recommend having a Mass said for your mother and maybe
a blessing of the house. It is also good to redecorate and clean
things out. Memories can haunt us. Sometimes it is necessary to make
a change. The living should not live with ghosts, even if just those
in the mind and heart.