Cusotdy and Third Party

My husband and I have been separated for 4 months. My oldest daughter wants to stay with her grandmother other than either of us. According to my STBX he wants the kids to stay with me but his Mother is saying she is going to hire a lawyer to see if she can get my oldest daughter. Question is how would the judge see this? I dont think its healthy that my cjildren be separated. I dont have a drinking problem, or anything else. She just wants my oldest daughter to stay with her. What would her chances in court be to get my daughter from me?