I know this because I'm well read enough and have delved enough into my own family history. Shiva can be sat when:
- A child or other relatives intermarries (This happened to the late Ruth McBride Jordan, nee Rachel Deborah Shilsky [Ruchel Dwajra Zylska] when she was even just in a relationship with Dennis McBride.). This normally doesn't happen if a child marries solely a "shvartze"--as the Yiddish pejorative was thrown about back then. This happens if a child marries any gentile who doesn't convert or is not accepted for conversion into Judaism.
- A child or other relative converts out of particularly Non-Messianic Judaism into--e.g.--Roman Catholicism (or other so-called "denominations of Messianic Judaism [Christianity]"), even if they remain Non Messianic and are merely Anusim. Sometimes, shiva isn't even sat and there's a rowdy hatefest where the convert's "death" or death is celebrated (See "Evel Rabbati", which takes work to find but is worth fighting. As the Anti Semites like to tripe on about and the Karaites note, this is what the P'rushiyin do not want you to know.). The P'rushiyin particularly hated Jesus, and revisionists like to pretend that another Yeshua is mentioned, but all one has to do is look at Bava Metzi'a 59b and the New Testament to get the idea pretty quickly.
- A child or other relative acts so despicably that the circumstances play out as if he or she is dead, would rather be dead, or would be better off as such.
- A child or other relative is in circumstances in he or she is dead, would rather be dead, or would be better off as such. For example, a child who is inevitably going to die of terminal cancer and not be miraculously healed may have shiva sat for him ot her early on.
Shiva can always be reversed for the living. For example, Ruth McBride Jordan did get back in touch with one of her cousins who she hadn't seen in years and who was in California. Even relatives who had sat shiva for her (viz. her aunts Mary and Bernadette) had contact with her, even though that contact was telling her that they sat shiva for her and she was to stay out of their lives (which Mary did)--as well as slamming the door in her face when she reached out to them for help (which Bernadette did).
In my own family's case, I have been in touch with Rusnak relatives before. Also, the Czerneckis did write to us once to ask for the deed to be changed. Shiva will also be reversed for Aunt Mary if she ever reaches out to me again (See #3 to give you an idea of what she did. All that I'll say is that, that took hutzpah to cut me off without at least telling me why she would--at least Ruth McBride Jordan's aunt Mary told her that she cut her off and gave her a hint as to why she did.).
Shiva cannot be reversed for the dead (unless they of course miraculously rise again).