From the start, we should know that this poem isn't going to be super-upbeat. An epitaph is something that is written in memory of a person, in this case, an old woman who has died. Generally, these epitaphs are quite sad, though every now and then, they have a positive message for those the dead left behind.
They buried her in the family tomb (1)
Think about the people who are burying her, and how they likely have other family members buried in the same tomb. Think about how being in the place where all those they have lost are buried would make them feel: deeply, deeply sad.
and in the depths the dust of what was once her husband trembled: (2-4)
Though there's nothing explicit about sadness in these lines, reading them gives us a very solemn feeling. We feel what is probably sadness mixed with joy in reading that this woman's husband awaits her and trembles when she joins him in the grave.
joy for the living is sorrow for the dead (5-6)
Now, we get down to the nitty gritty of sorrow in the poem. There are a couple ways to interpret these lines. One is that sorrow only happens because we experience joy. The other is that all joy ends in sorrow. These are the two interpretations we've been talking about, but please don't let us confine you. Run wild with the lines, and read them how you want! For the purpose of thinking about this quote, though, it's important to notice that "sorrow for the dead" ends the poem. That's a pretty dark and hefty way to end a tiny little poem like this.