They buried her in the family tomb (1)
At this point, we know that family likely had an important role in this woman's life. The family has a tomb, where each family member will be buried. This means that the "they" in this line is most likely younger members of the woman's family. Maybe, they're weeping as they let her down, or perhaps they are simply reflecting on her long, full life.
and in the depths the dust of what was once her husband trembled: (2-4)
This woman's husband helped her form the family that is probably helping to bury her now. We'd like to think they were deeply in love, but regardless of whether they were the biggest lovebirds of the century or just barely got along, they probably led long lives together, and became very close. They're now rejoined after being separated by his death, which was before hers. This poem makes us think that, wow, a reunion like this can be so powerful it can shake the dust of the dead.