This sounds like a tough situation and I'm so glad to see that you're trying hard to make sure everything is ok with your daughter. I don't think counseling of some kind is a bad idea. she might tell a therapist something that she isn't comfortable telling you, such as resentment for the amount of attention her brother needs.
I think that keeping her routine as normal as possible is definitely important, but I also think you need to make a point of planning some special things for her too. Remember that special does not have to be expensive, it just has to be quality time where she is the main focus. Go out for ice cream, have a movie night at home, bake cookies, etc. Let her know that you are still there for her.
Talk to her teachers so they understand how she's feeling and can keep an eye on her at school. Preschool teachers can be a great resource, as they have seen countless children react to the birth of a new sibling and can help interpret each kid's reaction. They can also make sure that she's being included while playing, or interact with her if she's choosing not to join in the other games.
I can only imagine how stressful and overwhelming it must be to take care of a sick newborn... I only know how stressful and overwhelming a healthy newborn can be! So this must be so much harder. It is understandable if you haven't had the same amount of time or energy for your daughter that you have in the past. But now that she is reaching out and letting you know she needs more from you, it's really important that you do your best to show her just how important she still is to you.