I think as others have said it depends on how far along the pregnancy was.
we have a rainbow baby. We have lots of bits around the house with her brothers name on, and relatives mention his name, she sees his name on decorations and cards. She kept asking who that was. Until she was 4 we would take her to the cemetery with us but stopped when she started to learn to read. She was six and a half, around the anniversary of our loss when we told her.
I wished we did it earlier, as in when she was 1 and so would grow up normalising it and before she can really understand, or when she was much older. The reason why we told her is because it is coming up to the 10th anniversary of the stillbirth next year and we do not want to hide our son, it would be impossible to hide and she has already been asking who he was.
My husband thinks we made the right decision, and my daughter has told us she is glad she got to know about her brother, but it is just so much for a little person, and for us supporting her with it. I am in two minds whether it was the right decision. I do not think we have ruined her life as other posters have said, but then she was not a twin.
She has told me she misses him but misses our cats more! We had a cat who died who our daughter knew her whole life and she understands grief, loss and death as much as a child can.
What she finds hard is that she has no memories of him. She is not distressed at knowing he existed. Before, when she was feeling sad (and did not know why) she would tell us she missed Nana (who is very much alive), and then she missed another living person, then it moved onto the dead cat and now it is her brother. I have no doubt she does miss him, I know she does, but his name is also attached when she cannot articulate why she feels sad or out of sorts, which we all get
there was also the emotional side of telling her, and supporting her with her feelings, but also we have to share our grieving with her and it takes it out of us e.g. we bought some flying wish paper and wrote messages to her brother (as well as our old cats and other relatives) and hers did not float as much so we had to deal with a tantrum because she is a child and wanted hers to float, and having to patiently deal with that when grieving ourselves. Because she is a child and wants to be involved and we want to involve her.
so I would say either raise it now before he can understand or wait until post 10, and work out how to manage your grief while hiding it if you do that.
have you spoken to Twins Trust? They do a lot of support work with parents who have lost a twin, might be worth having a chat with them as they will be someone who gets it, and there will be forums of people who have been in your situation.
sending you lots of love, unless you have been in the situation it is so hard to know. It seems the other people who have posted who lost a sibling seem glad that they knew.