Engines hanging from a wing are actually a good thing sometimes. The lifting force will try to bend the wing upwards, but the weight from the engines opposes this, causing the bending moment in the wing to be less.
Another good analogy is a tree. Trees bend in the wind, if they wouldn't do this, they would break a lot more often then they do now!
As for how much stress you can apply to the wing? In tests they apply loads to wings that make the tips move up over 20 feet I think, and the wing will still not break. There are some impressive videos showing these tests, you might come across them on the Discovery Channel.