if I remember well he's complaining about his brother-in-law ("beau-frère" in French) Bellegar that summons all kinds of creatures.
It's a bit different actually: Bellegar plays tricks with Robin's dead brother-in-law, but he isn't the brother-in-law himself.
In the ground, out of it again, in the ground, out of it again... Annoying things! Fifty six kills and it was my dead brother-in-law again each and every time. Bloody Bellegar and his bloody jokes!