Professional diplomats negotiate alongside trusted representatives

Would a ruler send a son-in-law to negotiate?

Probably not in the arrangement described: professional diplomats negotiate, accompanied by a politically trusted senior figure whose role is usually outside the formal negotiating team.

Professional diplomats conduct the formal negotiations in the arrangement described. A politically reliable senior prince or noble can accompany them without becoming a named negotiator. That representative takes part in meetings around the negotiations and supplies political standing. A son-in-law would usually lack the status needed for this particular role. Diplomatic practice varies across periods, so the example does not establish a universal rule.