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.