When it come to mental issues, there are really only two categories; those that require hospitalization and those that don’t.  If the sibling requires hospitalization, then the amount of involvement by your SO is minimal.  There may be the occasional consultations with the medical staff, but, by and large, the staff handles everything.

The other category are those individuals who are well enough to function on their own, but do require constant supervision.  These are the ones who will require the most from your SO, so be prepared.