Originally Posted by FlyGap
I would do everything in my power to relocate the filly. She is going to become a terror as she gets older and will certainly do further harm to your little SIL. Your husband needs to talk with the family, NOT YOU, and convince them to get rid of her. Your SIL is a child. She has no business being around a dangerous young horse without parental supervision. Obviously she does not have the training to handle a young horse, the common sense to stay away and listen to advise, and her parents need to be reminded of the potential dangers.
Great she got to experience the wonders of foaling, now it is time to have her either professionally trained or sent away to "someone who will love her and give her the training she deserves." ASAP
And welcome to the forum!
Meh, I disagree -- yes, it is the OP's husband's family, but she is the one who has witnessed specific events that have caused her to conclude the SIL is in danger. This is not one of those "It's his family, his to deal with" subjects, imo. It's not about who's family is who's - it's about who has the knowledge and experience and has witnessed specific situations that are raising big red flags.