Like most other people have already replied, there is no exact formula to know for sure if your sister's elders will have a memorial for him or not since ultimately it's her congregational elders' decision. I hope they decide to have a memorial whether it's at a Kingdom Hall or not. This is what happened in my family's case: I lost one of my older brother's when I was 17 and I had been baptized for only 4 months. The elders take into account many factors when deciding whether or not to hold a memorial at the Kingdom Hall. My brother was 26 and had committed suicide. He was a smoker, used drugs occassionally, was never baptized and didn't practice the Jehovah's Witness faith. What he did in his life doesn't matter to those who loved him but it matters to the elders since they consider the memorial as a time to witness to others about the hope of resurrection, not so much as remembering the individual. My parents were divorced, my mom sort of raised us a JW's but was inactive. My father stopped going to meetings years ago and lived his own life. Out of the 5 kids I was the only one baptized and one of my brothers was going to meetings but unbaptized. I don't mean to talk about my family, but I feel it's relevant since the elders took all of this information into consideration when deciding if they would have a memorial for him at a KH or not.
We as a family had at least 2 lengthy meetings with the elders to determine if they would have the memorial at the KH for him. We were shocked, considering we just lost a loved family member, that it was even a question whether or not to have his memorial at the KH. It was traumatic and painful. While we were trying to cope, we had to essentially defend my brother's life as a troubled but good person. The elders told us that the reason why having his memorial at a KH was even an issue was because if he practiced vile things it would bring reproach upon Jehovah. Ultimately, the elders allowed us to have the memorial at the KH since my brother had showed interest in studying the Bible shortly before he died. I hope that when your brother-in-law passes that the elders in her congregation are more loving than mine were. No one should have to attempt to cope with the loss of a loved one while at the same time trying to defend their lives and their right to have a memorial. My heart goes out to your family and especially your sister in this difficult time.