There is nothing wrong with having relationship with his sister. Having said that, I’d not speak to her about him. If my husband called my brother to discuss me in some kind of complaining way, my brother would hang up on him. That’s not right. My husbands ex was estranged from my husbands family, well also from her adult kids. She loved to call people to discuss my husband in a negative light. They’d not engage in it and eventually cut her off. You are estranged from your family. Your run a risk to get estranged from his family too. Do not involve his sister in your disputes with her brother