She's probably doing it for the reactions she gets. Sometimes, the best form of discipline is to completely ignore the behavior. If you are grabbing her hand and saying "NO" or "OUCH" or "GROSS" or whatever it is she is doing, for whatever reason she finds it entertaining. The best thing to do is recognize for a baby like your daughter that she is finding your reactions to the good behaviors more entertaining than the reactions to her bad behaviors.
Of course, this is very difficult to do. It is hard to keep a blank face when your baby starts to hit, kick, scream, pull your hair, or spit. If you are holding her and she does this, you can try to say no really quick and put her down. Once she's done, pick her back up and let her know she is being good. Play with her while she is being good. Leave her be while she is being aggressive.
It is hard for younger siblings to sometimes feel like they are being given enough time and attention, especially when both parents are busy. Often, the quickest and easiest way to get attention is to be aggressive. Keep a little journal of her naps, play times, bedtimes, meal times, etc. and figure out when she is most aggressive. Keep a little mental log of when the last time she played with you was. Perhaps all she needs is more reassurance that you notice her just as much when she is nice too.
