If these two sisters visit again, ask them nicely. Put them near a window, then take out the camera! Try to make it about getting to know them a little better, and not about the camera. Though your Speed will generate some interest, and a little surprise when you take it out!
It would also help to find a willing model that you can photograph frequently. It really does help to make a lot of portraits, and get comfortable with the process, and to see results. You've got plenty of gear to make interesting portraits.