The zoo has been working to introduce the two sisters with Luke for some time now in the hopes of establishing a proper pride, and it looks like they are succeeding. The cubs are doing well, and Shera is turning out to be a wonderful mother. And even better news: keeper's believe Nababiep may be pregnant and expecting again after a mating with Luke in May. Such exciting times!
While Shera and the cubs are not yet on exhibit (but hopefully will be by late fall), they can be viewed on a live webcam here and a short video of the mother and her cubs can be be seen at the Zoo's YouTube site (since I know the webcam might be getting a lot of traffice right now).
**UPDATE 22 SEPTEMBER**
Nababiep was pregnant, and expecting! She gave birth to three more cubs early this morning. That makes Luke, Shera, Naba, Shera's four (female?) cubs, and Naba's three. That's 10 lions at the National Zoo. Incredible. The pride is finally coming together, huzzah!