On Skyrim: Where do I find Leifnarr's Family?

This is part of the quest Flight or Fight, so you need to head to Heartwood Mill which is located west of Riften and speak to Grosta.

Heartwood Mill is almost due west of Riften, on the opposite shore of the large lake that Riften is on (Lake Honrich). There's a map here.

There is a known bug in this quest where Grosta may not acknowledge you found her husband if you find him prior to talking to her. As you're on a console, the options for fixing this are relatively limited. However, on PC the Unofficial Skyrim patch fixes this.

Alternatively, if you are on a platform (Xbox/PS) you can just kill the wife if you want to clear the quest from your list.