Yes ... I agree ... I've almost added your Ellen to my database as a daughter of Pierre Charles Courtois and Marie Ann Lejeunesse several times now but I keep stopping myself because there's no written document yet that says so in print ... but all things considered, it's got to be. She's definitely part of the family and thats the only place she fits.
But I haven't turned over every rock yet ... Charles died in 1875 ... there is a good chance that he owned property and perhaps there are some legal documents about how the property was distributed ... with Ellen still being in the area at that time she may be mentioned on such a document ... Also South Hero has a document that he had three children in school in 1823, so perhaps he also sent Ellen to school in Plattsburg about 1840.
I can look through Charles War or 1812 pension file for any mention of an Ellen/Hellen/Leonore/etc. I read all the documents in the file before, but not specifically to search for Ellen.
Also Ellen might have some sort of legal or town document that names her parents. Or one that names one of Charles other children as definite sibling. The way I look at it, if we're going to reply on circunmstantual evidence, the more circumstantual evidence we accumulate, the stronger the case.
I knew a marriage in Canada was a long shot as someone would have found the marriage years ago, but we can't rule it out unless we look. Now it's definitely out of the question.
Perhaps if we get photocopies of the actual baptism record as written in the church register, the priest may have specified Julie Courtois' relationship to the family when he recorded the baptism.
I'll try to post the info from the other Mire/Barry baptisms.