First of all, I don't blame you! Once something is given to you, it's yours! (even if it came from your mother) If it were me, I would explain that since you've had it for the past year, you've found it very useful and that it is being used for your belongings. I wouldn't bring it up to them and if it comes up again, I would explain that you are using it and you don't have a place for the items in it if it were gone.

If they say they are coming to get it, I would not empty it, but I would explain that if a time comes that you no longer need it, your sister will be the first one to know about it and she can decide then if she still wants it.

Hope this helps!