You have several options to help your family. It sounds like you are doing your best to help in this situation and you are a good support for her grandmother. You and her grandmother have seen a dramatic change in her behavior at home and at school. Ask yourself these questions: What has changed at school? Does she not like teachers this year? Is she so far behind she feels she can not catch up? Is she able to ask questions to get help? Does she need help with learning some good study habits? Is she having friendship problems? Has the environment at home changed in anyway? Does she feel supported and encouraged while at home? Can her elder sister talk with her about her choices or encourage her? Have you sat down with her to discuss your feelings about the choices she is making for herself? Can you encourage her to talk with you about her feelings as she seems to trust you? Has she admitted to stealing the money? Has she earned any negative consequences for her behavior? (If she does not complete her homework, does she earn a consequence? If she steals from others, what is her consequence?)
 
Remember that consequences change behavior. If she makes good decisions for herself: she should be rewarded (a positive consequence). If she chooses to not to her homework, steal, lie, etc: she will loose some privileges (earn a negative consequence). Encourage her grandmother to set up some expectations for her behavior. It sounds like she is capable of getting good grades, and she needs to hear this. Try and talk with her and ask her if she knows why she is making bad decisions. She may need some attention. Sometimes, kids make behavior choices based on attention. And she seems to be seeking attention negatively right now. You can help her shift this to positive attention seeking, but making better decisions. Have her come up with a plan on how she will complete her homework (this could be a consequence). Talking with the teacher can also be very informative and helpful. Can she take responsibility and talk with her teacher about some strategies or extra work that she can complete to bring up her grades?
 
You did the right thing by contacting the police and let them talk with her about her choices. Just by sitting and listening to what the police have to say, you can praise her for participating and being there. That is a positive choice that she made: just to stay in the room and be respectful to the police. She may have to pay restitution. Are there some extra chores she can do to 'earn' that money back?
 
Think about teaching her some skills: respecting others privacy and property. Talk about this at the dinner table, with the whole family. You will really get a sense of what kids are thinking if you prompt this discussion. Have them think about famous people that have gotten to real legal trouble or trouble in their relationships for their poor choices. There are many examples.
Talk about trust and what makes good relationships. Trust can be lost very quickly, but to gain trust with others takes time. She will have to earn trust back with her grandmother, and this will take time. Have her be in charge of the 'sweet snack' items in the house. She will have to do a daily tally of how many are in the pantry, and who are eating the sweet snacks. This strategy is a good one for kids this age, as the adults can check up on them. Have her check the chores completed by the other kids. She will have to report to her grandmother and let her know if the chore was completed.
 
Keep being a good support for her, she is very lucky to have you in her life! If you feel need to talk to someone, please consider calling the Boys Town National Hotline at 1-800-448-3000. We have counselors available 24/7 and we talk with kids, parents and caregivers about various issues. You can call, the grandmother can call, or even your cousin's daughter. Thank you for reaching out for help.