One thing you can do (I did this with my great grandmother's siblings and parents) is make a copy of the relevant Bible pages and attach those to the profiles of the people indicated, and then cite "Smith family Bible" (or whatever) as a source. Often, once you have the dates and names, you can find additional "official" sources (although for children who died young, etc., there may be nothing else out there).
As to whether there were other siblings, it's certainly possible that if they died very young, people did not talk about them any longer by the time your great aunt came along (assuming she was the youngest, for instance). If by "collections search" you mean something like the Family Data Collection, that's nothing but what people have reported and no more accurate than others' trees - I would take it as a hint for a name to search, but nothing more.
For how to source you great aunt's information, I would list it in the "description" section and maybe add a "story" that says what you learned from her, then attach it to each relevant person.