To be very honest, you will not get custody unless the kids came willingly to your house. Can you contact the kids through social media for contact? Facebook, instagram, twitter? Can you send a letter to kids by way of another family member and see if they will give it to them.
You can contact the kids school and get information that way. Use the form from this site to ask for all their records. It will have their address and phone number and then send a letter to them asking if they would like to communicate with you. Offer to meet at a counselors office ? Public place ?
At their ages, they will not enforce visitation so if they do not want to see you , it will not work. Some have asked for visits at therapist to get through some situations.
Court- You could file at family court near them and ask for a modification of the visitation (parenting) plan. Ask for "mother requests that court ordered therapy visits be ordered between her and the girls to re-connect and a
parenting plan be made that the courts/therapist see fit".
How far away are you from them? If you are states away or long distance, money for travel comes into this all too.