I don't think writing a sentence over and over again is necesarily indicitive of a mental illness. People write positive affirmations over and over again. As P said, it just sounds like your sister is heartbroken.
She sounds really heartbroken and unhappy, and hasn't gotten over it. Anyone can feel grief over losing someone who they fell in love with and expected so much, and that includes people who aren't mentally ill. Have you asked her how she feels or what it is like for her? Does she have friends she can talk to and listen to her or in the family she can talk with? Can she talk to you and what do you say to her? What Weaver71 says is so true. If you are looking for someone to share their doctor with you, it isn't really possible. For one, no good doctor will feel comfortable doing that since they can't practice in your country and they can't treat someone they can't see and talk to in person.