Is your sim's father's tombstone available at the Mausoleum when you Manage the Dead? Then, at the very least, you could place the grave somewhere and mourn.
That was always one of the things that bothered me about standard story progression. One would think that your sim would mourn family members and close friends outside of the household, yet for some reason that bit of realism wasn't included in the game. There are mods out there that will perpetuate the Mourning moodlet for family members outside the household (Twallan's StoryProgression being one), though I know that mods aren't for everybody.