My grandparents had one and it was so painful they didn't discuss the stillbirth - I was stunned to learn about it in my mid-20s or so. (My grandpa was sick by this time and both felt, I think, it was best to mention that it had happened at least.)
(I actually wanted to have a burial for her and got paperwork ready to exhumea nd rebury, but it would have been too hard to figure out which grave was which where she was buried so we didn't. "We know she's in Heaven, that's what matters" is what my mom figures.)
I think from a family history prespective it's important but I'd put it in your own profile or wherever and not in the tree. It might actually make a nice little ceremony for his/her living twin when they are old enough - let *them* honor that stillborn child by being the one to add him/her to the tree.