Hi Patti,
Most often, when we are dreaming, each character in the dream represents an aspect of ourselves.
So if we dream of a tree, a bucket and a toddler, we are most likely dreaming of the tree, the bucket and the toddler as representing something within our psyche.
A shadow person is someone you can't see. You can't even tell if it's male or female (I noticed you called it "it.")
That may be representing some aspect of yourself that you've yet to understand.
Why do adults wear diapers? Most of those reasons are related to aging, health problems, and being unable to control certain bodily functions. A diaper protects against the mess of the inability to release something at the appropriate time.
I would see the key elements in this dream as:
Appropriate timing
Releasing something
Unknown aspects of self
Since you woke up thinking of your sister who had passed, you may also ask yourself if she had anything in common with the character in your dream.
Anyway... that's where I'd start if this were my dream.
And I'm sorry to hear about your sister.
