“Where does this idea come from that wearing tsniusdic clothing is less of an obligation then keeping kosher? “
I don’t believe it to be less of an obligation – only that it is NOT equivalent. If someone eats treif in his home, you cannot eat in his home. Period. If his daughter wears a skirt that covers the knee but not when she sits down, do you truly feel she is treif? If so, I respect where you are coming from, but there is nothing further to discuss.