We have a mom and a daughter that have always been together. The daughter (Penny) what handled constantly when she was little and the mom (Tera) was always very cooperative when it came to us messing with Penny.
Now Penny is 3 or so, and there haven't really been any problems. Tera was kept at my aunts house for a while without Penny and there wasn't much change in behavior in either of them...no more than when you seperate any horse from their herd.
When they take Penny and Scarlett (another mare about Tera's age) out to ride Tera does get a bit panic-y if she is left in the pasture alone, but I don't know if that is b/c of her daughter, or just b/c of being left alone. Could be either. We have had horses that do that in a herd of unrelated horses. Tera can't be ridden really any more so I'm not sure if that would make a difference.
She is a pretty little girl and good luck with her!!