How do I stop crate aggression and territorial behavior?

We adopted a pit-mix almost 2 months ago; my fiance and I work overnights every 8 days; we've had my sister and FIL come over to let him out; they are not strangers, they stop by to pet and play and feed him every other day. The longer we have him, the more aggressive he's becoming towards them letting him out of his crate. He's fine when we are around, but will lunge to bite (hasn't actually bitten yet) when we aren't, especially towards FIL. Professional trainers are expensive, especially if we need to go long-term. Not sure how to correct behaviors when we aren't around, while we actually are around. Any suggestions, especially from pit-owners?