Completely normal. They are learning SO MUCH right now that their brain is on overload and they don't want to miss a thing! Is she learning to crawl, scoot, etc? Sleeping through the night is a milestone just like crawling, walking, etc.
6 mths is another growth spurt for BFing. Since she is 6 months, I would start introducing more solids into her diet. You can actually replace the cereal with a meat and a veggie, fruit. I made all my baby food www.wholesomebabyfood.com, but you can buy them jarred if you prefer.
My son didn't sleep through the night until he was over a year old and we had to do cry it out for a couple of days. I didn't feel that CIO any younger is beneficial because that is their form of communication. I BF him until he was 16 months old.
Hang in there. Yes, it is hard with 2 kids getting up at night (I went through that for a few months), but just be consistent in your routine, continue BFing and things will get better!