It's good to work on this NOW rather than assuming they'll grow out of it or just someday begin to see you as one in authority & who deserves respect. To expect children to give respect means to treat them respectfully as well - which I assume is already the case. Not only does this affect them in the home it will also affect them in life in general, not learning how to properly listen to those in authority (teachers, employers, etc.).
I would try to think of a few creative ways of changing up how you communicate to them on a regular basis & then start fresh with a new method, not letting up on it for several weeks. And using the new method EVERY. SINGLE. TIME. either one of them shows you a level of disrespect. Once they start falling in line & you can see they are starting to understand the importance of listening, giving respect, obedience, etc., then you can back off & employ the new method at various times. If they start going back to ignoring you, then reimplement the new method every single time.
Some suggestions might be to take the younger one by the hand & lead him to the couch while you take the older one by the hand & lead her to the kitchen table or some other separate place. Tell the older child what needs to be said, then walk over & tell the younger child what needs to be said. If they don't obey or pitch fits then there will be a consequence appropriate for their age.
Or if there's a TV on or a game being played or some other distraction besides just the 2 of them talking, quietly walk over & turn it off / put it away / close it up & then wait for them to be quiet & looking at you. Then start talking to them about whatever it is that needs to be said.
I think you've already received some excellent suggestions that can be new methods of dealing with it, depending on their particular personalities & behaviors. It's going to be tedious, exhausting & frustrating, but if you stick with it you will definitely see some changes in them, if not immediately then very soon! And that should be an encouragement to you, that you ARE the mother, you DO deserve respect & your children WILL listen to you, whether they think they need to or not.
Good luck!