Your little girl is being a normal 2-3 year old. That age is all about independence. They are beginning to think for themselves and want to "see" how much they can do. An independent child can be a challenge, but on the positive side they can also be a lot of fun. My youngsters never hit the terrible twos, but 3 was a different story!
There are many techniques that you can use to help your current situation, but the most important thing is to be consistent and to let her know that there are rules and she is expected to follow them. Two of my three are very independent, and we have tried to enable them to do things for themselves beginning at a young age. In our experience if they can feel in control of certain activities but understand that does not apply to everything, it makes a real difference.
For example, during your bedtime routine, you might let her choose the book you read and have a chance to "brush" her own teeth before giving you the final brushing. You might create a poster that she can add stickers to when she handles herself appropriately or one she can color in. We had a clown juggling balls and each time we brushed and got in bed with no fuss, one of the clown's balls was colored. Once the paper was full, the child was rewarded.
Time out or loss of a favorite plaything is an effective punishment. It is not easy, but it is necessary. If she won't stay in time out, you keep taking her back to the spot until she does. Pick a particular chair or corner and use it consistently. Eventually--after much protest and many tears--she will learn how it works.
Remember that disciplining your child is actually another way to show your love. It's not about being nice vs. being mean. You have rules for healthy, safety and teach your children how to become successful adults. Someday they will appreciate your efforts--even if they are less than thrilled in the moment.
As for your son, I wouldn't worry right now, but you should keep an eye on his development. My son is mildly hearing impaired, and we identified his problem because he wouldn't speak much--however, that was at age 3 not at 15 months. YOu can encourage your little one to talk through play. If he wants something, make a game out of trying to figure out what it is. If you are outside with him and he is going down the slide, make that a word game as well. It really works well!
Good luck.