N.L.
Couple of quick things I want to mention. I'm in a bit of a rush and I am a therapist so it may be all a bit technical but look up the terms/theories to educate yourself on it further and it will help guide your intervention.
Seperation anxiety has a basis in a couple things. For one, your daughter needs to feel secure that her needs can be met by more than just mom. Work her into it by being present while other people provide her with food, etc. For two, at this point it has probably moved beyond that and it's really about Object Permanency. This has to do with her being able to retain an image of something beyond its disappearance. This is why Peek-A-Boo and the whole baby dropping an object to see you pick it up game is important. But again, she's a bit older now. Maybe hide-n-seek.. maybe with objects like fav toys then mommy, brother, etc. Can't really think it all through now but if you understand the basis, you might get creative on how you continue to address this need for her. Also of course you can give her a transitional object. Lots of verbal reassurance. You can use her age to an advantage and give her a digital watch (if she can't read a clock yet) or a timer that you set for when you'll be back. And she will need to learn self-soothing techniques to manage her anxiety. It is definitely important that you have her in therapy but I do suggest that you continue to look for ways you can work in conjunction by trying some interventions yourself.
Let me know if you need more suggestions to get you going.. gotta run, good luck! -N.