It is natural that you are torn. You want to be supportive, but you are a new bride. You understandably want your husband’s attention. Your husband and his sister may have long-standing habits that are continuing because they are in their childhood home. You may not be comfortable with the customs of his family and yet you have to somehow fit in with things as they have always been. It’s very difficult.
Every marriage is a cross-cultural experience. Each of us comes to our marriage with expectations and habits that the other may not understand. My best suggestion to you is that you start talking to your husband about how difficult it is for you to blend in with his family’s style. As his wife, you want to be the one ministering to him when he is hurt. As his wife, you’d like him to spend more time with you and less time with his sister — not because you are jealous but because you love him and want couple time. Suggest to him that he probably can’t be more helpful to his sister with her problems since the problems still continue. Perhaps she and her husband need to see a counselor instead.
If you can speak from your love and your need for him instead of with accusations and suspicions, you are more likely to be successful.
I wish you well.