You have to talk to the parents yourself. Stake it out. Walk right over when they are dropping off or picking up. All the Aunt said- if anything- is that the neighbors are a-holes. Let them know you have seen the behavior yourself and spoken to Granny Numnutz and Auntie Lamebrain. Tell them your daughter has never complained, and you wouldn't have even known about it had you not caught them redhanded and heard from your sister in law.
Tell them you'd hate to have to blow this out of proportion when they should be able to handle their own kids, but if your daughter continues to get harassed in your yard, you'll have to report them for repeated disturbance. There has to be some form of harassment you could cite, calmly with a confident tone all like,
"Hi, I'm the neighbor you heard about, Auntie Jerkface (use her real name of course) told me she would speak to you about the yelling I have seen your kids do to my daughter when she's in our yard. Did she mention it? Just so you know, my daughter has never complained, this is me talking. Obviously, you should be able to handle your kids or leave them with someone who can, so it would be a little ridiculous for me to have to report this harassment elsewhere since these girls are minors. Hopefully it won't come to that. I'm sure you can handle this yourself. Thanks."
And I would probably call someone if it keeps up since the neighbors are disturbing you guys and have been asked to knock it off.
I would google "minors law" etc and see what you can find out about minors harassing your daughter in her yard, and who's responsible, and see what term you could toss around to them before you say something.
Sure, this will make them even more hateful and get them really spewing vile things about you in their home (like they don't already), but they MAY JUST threaten them to really knock it off, and it's not like the girls are your friends now. The other alternative is teach the kids they're allowed to harass people and they get away with it.
I really put myself in your shoes and pictured my daughter out there getting yelled at by some brats at their grannies every day, and unless they were armed gangsters, I'd be in that mom's face in a heartbeat every day.
I agree your daughter should shout back and retaliate in general in a fleeting episode at school, but for a home situation, you need to handle it, and model to your daughter how to stand up to people and not get overpowered.