There's not really a good way to prevent her from doing it. You can only get a protective order if you meet the criteria to obtain one which includes family violence which you have not mentioned. Even if she were to file for temporary custody/guardianship, she would have to show that you and the xhildren's father are unfit parents to have the kids. Her dislike of your boyfriend is not enough unless she has some strong reasons like drug use, he's violent, etc.
Unfortunately, anyone with the wish to do so, and the funds to carry it out, can file a lawsuit. You cannot prevent that. What you can do something about is to assure that her efforts fail by being a good mother to your children. No one, grandparent or otherwise, can take custody from the biological and/or legal mother without a showing that the mother is not fit to care for her children. Making up false accusations of family violence is not the way. If she files anything at all, you will need to retain an experienced custody attorney immediately. Do not wait until a court date is scheduled. You need an attorney to file proper pleadings and prepare properly for court.
I am exclusively a family law attorney, practicing primarily in the metro Atlanta, Georgia trial courts. However, I handle appeals from anywhere in Georgia.
Sign up to receive a 5-part series of useful information and advice about child custody law.