[Pontiac]

My sister had no idea that I would immediately know who this family and half-sibling were, due to where I live, and having had contact with the family in the interim years due to strange coincidences.

Your sister took it upon herself to make contact with this half-sibling without informing the rest of you?

Seems rather thoughtless.

At least you would think that she would want to know what the rest of you knew about this development.

[NEMDF]

My sister told the rest of us after getting the new half-sibling’s okay to tell us about the new half-sibling’s existence.

[ifinnegan]

“Your sister took it upon herself to make contact with this half-sibling without informing the rest of you?”

The sister and half-sibling both agreed to any theoretical contact that could arise by doing the DNA test and allowing sharing.

Initial contact is simply seeing the list of DNA relatives.

23 and Me has messaging like here at FR if people who match want to write to each other.

In this case it’s doubtful the sister knew in advance she would have a half-sibling match.