"So, you no longer want to speak to your brother-in-law."
Translation:Donc, tu ne veux plus parler à ton beau-frère.
It has the same overall meaning, but "avec" is "with" and Duo wanted "to".
Indeed, the next is accepted by DUO: "Donc, vous ne voulez plus parler à votre beau-frère"
You can also say, "Donc, tu ne veux plus parler à ton beau-frère." As long as you have the same pronoun, it should be fine. Personally, I think that the "tu" translation fits the context better than the "vous" does. You normally wouldn't say this to someone that you use "vous" with or a group of people. The "vous" translation is still valid and accepted, I just prefer the "tu" in this context.
It is grammatically correct. I mean you probably wouldn't be using this sentence when speaking to authority and certainly not multiple people (brother in law is not plural). So, it might all be in the context.
Edit: I see it now. You either need to use the "vous" or the "tu" translation, you can't mix them up
What about the lesson where we were taught that donc cannot be used at the beginning of a sentence?
The negation words "pas/plus/jamais/rien/personne" always come after the verb.