HollyD:Her biological siblings will be too curious to stay upset forever.
Except for one brother, who is his own evil twin, I'm in no hurry to add to or to replace any of my siblings. Just because you envision yourself feeling a certain way doesn't mean that all of the other people will feel that way, also.
There is nothing to be gained from this. If she has some freak illness, which is unlikely and is not alluded to in her letter, she knows who these people are. If genetic testing at the time of her freal illness indicates she's unrelated to those she knows as family, she can contact the at that time.
They aren't like a woman who gave up her firstborn to adoption, and has been pining for her ever since. (If any do. I don't know.) These people, as far as they are concerned, have their sibling. This is the person they know and probably love. At best, a relationship with someone who "should" have been their sister would be awkward.
Other than genes (if that) and their mothers having been in the same hospital, they have nothing in common. No common background. No common history. No common friends and experiences. If they felt required to have a relationship with this woman, at best they'd be stilted. At worst, they'd be suspicious of her motives.