Linemen aren't fiber techs, and at least around here, very few have a "ditch witch" (99% of power lines are above ground.) You'd bury a fiber conduit, not a bare fiber jacket.
You could also go with LRE (long reach ethernet), or DSL repeaters -- "ethernet extenders".
You could have your in-laws string whatever from the utility poles, but the first time anyone else sees that or has to fix anything, it'll probablly be removed. (and a tidy fine show up.) I've thought about this route myself to get broadband over a creek to my grandparent's house (where my aunt now lives) -- I'd only be attached to 3 poles.