How can I counsel my sister after her boyfriend has died.?

Although I am very sad about it, their relationship was awful. He was physically and emotionlly abusive with her and he was a heavy drug user. He died of an acute infection and his death was unexpected (23 yrs old) and has two young children from another relationship. My sister is very much their surrogate mother, because their real mum is also a heavy drug user. I know this sounds horrible, but I can't really feel sad about his death because of how he treated my sister. I don't really know how to approach this situation.