The MacNab family lives on the lands of Lallybroch estate, and they treat Jamie as their landlord. They pay him for use of that land, but that's not all—they also expect him to protect them.
Grannie MacNab (that's all we know of her) asks Jamie to give a job to young Rabbie MacNab, mainly to keep him away from his drunk, abusive father, Ronald. Jamie does so, but Ronald doesn't give the boy up without a fight. Jamie beats up Ronald behind a barn… and drunk daddy later retaliates by turning Jamie into the Redcoats.
This family shows us how different people, even members of the same family, can have different loyalties. Grannie is looking after her grandson, while Ronald is simply looking after himself. As a drunk, he's selfish and prone to angry outbursts… and actions that almost get Jamie killed. The consequences for being disloyal to the laird of Lallybroch are severe: Jamie's pals burn down Ronald's house… with him in it. The lesson here, we guess, is don't burn Jamie Fraser. He burns back.