My Mom went through that same thing as a woman/Mom, and with my sister. The twin was born stillborn... the reasons for which are unknown "why" but the deceased twin was much smaller and not developed as much.
In my Mom's generation, they did not have the technology that we have today.
My parents, when my sister was older (not adult aged, but younger in childhood when they felt she was mature enough) did explain that to my sister. My sister... still treasured her twin. Its normal. My parents did not hide anything from her, but explained things honestly.... per her age. My sister, grew up always "feeling" like something was 'missing' even prior to her knowing the situation...and she'd "cling" to friends and not know why.
...and yes, Twins are known for having a "sense" of their other twin, existing or not. My sister as well, now has a Guardian Angel, looking after her. This is her comfort and special way of thinking of her twin.
For a bit of time, my sister sort of was angry at my Mom about it... maybe thinking it was her fault or her not taking care of the baby etc., or she "blamed" the Doctor for it. But, logically, she knows there is nobody to "blame." It is just her coming to grips with the whole thing.
It is no problem now. She can talk about it normally. But she is always cognizant that she is/was a "twin." Although she does not obsess over it nor does it impact her ability for daily living. It is all "normal" for her... a part of her normal life.
It is up to you if you tell her. But always keep in mind the emotional and maturity of the child. There is no predicting how a child/person will react to that.
The reason I was born... is because of that. My parents always wanted 2 children.
All the best,
Susan