Well, to be fair, since you are in absentia, the realtor is doing at least some amount work, so you can't expect her to fork over the whole broker co-op, even if she is a family friend. You didn't expect her to work for free, did you? I'm sure neither did she, so of course she registered herself for the broker co-op.
Even if you did do a lot of the legwork here on TI, you don't have much recourse, legally or even ethically, since you only found out about the rebate after the fact. Plus, the realtor browbeated you into looking harder at Orchard Hills when you were leaning towards Legado. So actually, the realtor did do her job.
I'm just surprised that with all your due diligence here on TI, you didn't see the big introductory post REBATE ON NEW HOME PURCHASES!!! Its even highlighted as a sticky and has 22,000 views.