Since this sounds like a group feeding situation (correct me, please, if I'm wrong) just a word of caution -- if they chase her other than eating, it may be that she is not getting enough to eat anyway. It could be they are so preoccupied with eating that they allow her in to eat, but then want her gone when the food is gone. There's nothing to say she's not getting her fair share, but let's face it, when it comes to food, few horses are, shall we say, "gracious."
Sometimes a horse isn't seen as underfed until it's obvious by the horse's condition weeks later. I'd make sure she got to eat alone, because eating isn't a social occassion for horses. It sounds like they are tolerating her.
In my assessment, with the little we know, I'd say this: the weanlings are not instigating any of the chase since weanlings are pretty timid when it comes to confronting an adult who isn't mom; the stallion might not see her as a "mare" if she isn't giving off proper scent. If it's winter where you are, she's in anestrous and not producing hormones. If he doesn't "smell" that she's a mare, he may see her as competition. The other mature mare might be protecting the foals, even if they are not hers. I've had all of these situations, so I'm not just talking out of my hat, as they say.
I'd move the new mare to being alone with the mature mare. Maybe put them in a fence that borders the run for the stallion and foals. Let him get to know her thru the fence. Let the older mare get to know her without protecting the babies. Eventually, they could be put together, if all goes as it usually did for me. Good luck with it.
P.S. I would suggest that weanlings definitely be fed alone (by each themselves or as a group of weanlings) especially for their first winter. They cannot compete with adults for the chow. Weanlings, (and mine were always on a full ration of feed before they were weaned) usually eat slower than adults and winter can play havoc on those litte'un anyway, even with full food. I don't mean to be critical, I just believe that would be better for them.