Sister insults parents and does not pray
7-6-2017 | IslamWeb
Question:
Assalaamu alaykum, Shaykh. I have a sister who is very bad. She does not pray, she treats my parents as animals or even worse, and so on. One day, I heard her say an insult. I think that she insulted my father, but she denied that she insulted my father and said that the insult was directed to me. It was strange that the insult would have been directed to me because at that time I was not even with her, and I was not having any conversation with her. So I accused her, saying that she insulted our father, without being 100% certain. I was accusing her in order to put her under pressure so that she would confess that that word was directed towards my father rather than myself. Was it haram to accuse her of insulting our father? Notice that I have not confessed this to anyone to avoid problems. I merely spoke to her and accused her in private, but without being 100% certain that that insult was directed towards my father; let us say that I am 70% certain of that.
Answer:
All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad 
is His slave and Messenger.
If your sister is as you described, then it is not forbidden for you to accuse her or think evil of her.
Al-Qurtubi 
said in his Tafseer (exegesis), “Most scholars are of the view that having bad assumption of someone who is apparently good is not permissible, but there is nothing wrong with having bad assumption of someone who is apparently evil.”
You must give advice to your sister and frighten her of the consequences of these abominable violations that she commits, especially abandoning the prayer, because the prayer is the most important religious matter after faith, and there is no share in Islam for someone who abandons the prayer and neglects it, as he is more likely to abandon other obligations.
Abandoning the prayer because of not believing in its obligation takes a person out of the fold of Islam, without any difference of opinion among the scholars, and abandoning it out of laziness is considered as disbelief that takes a person out of the fold of Islam by some scholars as well.
Also, undutifulness to her parents in the manner that you mentioned [insulting them and disrespecting them] is one of the grave, major sins and one of the causes of the wrath of Allah and His urgent punishment.
We ask Allah to guide her and enable her to repent, as Allah is Accepting of repentance and Most Merciful.
Allah knows best.