as its falling on deaf ears ,perhaps the time has come to take stronger action,could she not sign them over for rehoming?
Our daughter had to cope with a new baby and two horses ,she never let them get into such a state .
Its unfair to the horses and deep down iam sure your sister must be feeling guilty about the whole situation have you asked her?
If she doesnt care then she shouldnt have horses.
Any chance of her having an extra hand even if its to look after the baby whiile goes and sees to the horses..

any horses in that condition needs sorting out dont leave it till its too late the RSPCA wont mention your name so she wont know it's you