Its just the easy jokes doesnt need too much effort...
We have a saying in English, that "There is many a true word said in jest". basically it means that many times when we are "joking" about something, there is actually some reality behind it.
And it is the case with David Knopfler. To me, he is someone who made a career out of having a famous brother.
I have listened to some of his music and it is terrible, just awful. It never would have been released if his name wasn't Knopfler.
I can't even get excited about his status as a founding member of Dire Straits, as I know when I listen to those great records, he isn't even playing, or is mixed way down anyway.