Add more flour! If you add 1/3 cup of flour and the streusel is still sticking together and won't crumble at all, add more. You may need more than 1/2 cup total. Add 1 tbsp additions of flour until the streusel starts to crumble when ran between your fingers. It shouldn't be so crumbly that it won't stick together, though. If you squeeze a handful in the palm of your hand, it should stick together, but if you then take that handful and run it through your fingers it should crumble.