Hi C.,
I would suggest your sister look into some occupational therapy for him regarding his writing. Depending on where you are, the school should initiate this (perhaps at the request of your sister), and in many cases the cost is covered through the school system. This could help tremendously with his writing and the earlier the assistance, the easier it will be for Colin to learn how to grip a pencil properly and be on track developmentally with his writing.
Regarding his behavior, your sister might want to review his diet - are there things he's eating for breakfast, snack or lunch that he doesn't eat at home in the afternoon or evening? Allergies can impact behavior in a big way and behavior is often related to dietary issues. Secondly, your sister may want to request an impartial professional to observe Colin and his class over a period of a few days to a week at different times of the day. If there is a school psychologist, this would be his/her job. This could help identify why Colin behaves the way he does at school and/or what is going on around him making him act so differently than at home. It could even be related to his writing. If he is being criticized for the way he writes or constantly being corrected without assistance, this can affect a child's self esteem. My oldest felt like everyone else in his class was reading when he wasn't yet there. His teachers had many issues with his behavior in class and there were problems at home too. Even though he wasn't being criticized at all, he felt bad himself. Almost "out-of-the-blue" reading clicked for him. Almost overnight his behavior improved at school and at home. He still has issues, but not nearly at the level as before.
I've rambled on quite a bit. I used to be in education, so these issues hit home with me! I hate to see children struggling, especially when there are usually ways to help them!
Good luck figuring out how best to help Colin!
W.