I really don't know what to make of this. I can see both sides. Parents look out for their/and their child's best interest just as we have to look out for ours.
It seems like the mom has a preference for home daycare but wants the older children in a more structured environment. I assume that since the other DC isn't taking her infants then she perhaps only takes older children to run a preschool type program?
It really doesn't sound like the mom is trying to be rude...despite her actions being somewhat rude! If she is a good client and her children are enjoyable and everything else is working out then I would perhaps keep taking her youngest children with the understanding it is a short term contract. That's not ideal of course...but it's a space filled and a good family filling it.
By telling her you would leave it up to her to decide if she wanted to contact other parents you did just that, you left the decision up to her and she put her best interest ahead of yours. Perhaps if you had made it clear you would be upset by this and it would impact her ability to keep her youngest with you she may have opted not to contact them.
How easy is it to fill a space where you are? Do you easily have a space available for her next child to fill? I would consider that before committing. If you have a space to fill and have been having trouble filling it by all means take her child. If you don't have a space or you do but have many families looking at the space then take the best option available.
It's hard not to take it all personally but it sounds like this is a mom that is doing what she feels is best for her children. It may be not be in line with what we would choose to do, but she doesn't sound malicious.