Sleep Issues

So I guess I got a little cocky. I've been bragging every chance I get about how both my girls, sweet angles that they are, have slept through the night (STTN) since they were 3 months old (2 1/2 for Olivia). Yeah, they were horrible nappers but hey, they've STTN forever! They would go to bed between 8:00 and 8:30 and wake up between 7:00 and 7:30. I would hear them on the monitor talking and I could take my time and brush my teeth, make coffee, make their bottle, etc. and it was no big hurry to get in there to get them. Then they turned one and started drinking whole milk instead of formula and less of it. And Olivia started waking at 5:00 or 6:00, hungry, and of course she usually woke Kayley as well, although not always. Kayley, however, has consistenly woken up at least once during the night for some unknown reason. She goes right back to sleep when I pick her up and rock her, but still. Lately, Olivia has managed to start sleeping until 6:30, but that's about her limit. And instead of waking up happy and talking, she wakes up crying every morning. Plus they're going to bed later, like between 8:30 or 9:00. Sometimes Kayley doesn't go to sleep until 9:30 and then Olivia wakes her up at 6:30. Poor thing. The good news is they have been taking really good naps, usually 2 1/2 to 3 hours. But I just don't like the 8 1/2-9, maybe 10 at the most hours of sleep every night when they should be getting around 11. I really don't know if it's hunger or teething or what, but I'm going to try giving them some cereal or yogurt or something right before bed, and if that doesn't work I'm going to try moving Olivia into our room at 5:00 or 5:30 when Paul gets up for work and either giving her a bottle or some mum-mums or something, or just having her in there so she won't wake up Kayley before she's ready. On the other hand, if Kayley starts getting up an hour or so after Olivia every morning, their naps will not overlap very much. Ugh. I don't know what to do but hopefully I'll figure something out.