Praise be to Allah.
If the situation is as described, then undoubtedly this comes under the heading of transgression and mistreatment of others, for which there is the fear that the doer will be punished in this world and the hereafter. What is appropriate for you to do in this situation is to strive hard to put up with your sister’s mistreatment and bear it with patience, and not respond to her mistreatment in kind, as much as you are able to.
It was narrated that Anas ibn Maalik said: An old man came wanting to see the Prophet (blessings and peace of Allah be upon him), but the people were slow to make room for him. So the Prophet (blessings and peace of Allah be upon him) said: “He is not one of us who does not show compassion to our young ones and show respect to our elders.” Narrated by at-Tirmidhi (1919); classed as saheeh on the basis of corroborating evidence by Shaykh al-Albaani in as-Silsilah as-Saheehah (5/230).
You should also pay attention to the ties of kinship between you, uphold those ties and show kindness to her, for this is the true upholding of ties of kinship that is required according to Islamic teachings. True upholding of ties of kinship is when a person responds to the mistreatment of his relatives by showing kindness towards them.
It was narrated from ‘Abdullah ibn ‘Amr that the Prophet (blessings and peace of Allah be upon him) said: “The one who upholds ties of kinship is not the one who responds in kind; rather the one who upholds the ties of kinship is the one who, if his relatives cut him off, he still upholds ties with them.” Narrated by al-Bukhaari (5991).
Ibn al-Jawzi (may Allah have mercy on him) said:
You should understand that the one who responds in kind is the one who, if his relatives treats him kindly, he does likewise. The one who upholds ties of kinship for the sake of Allah (may He be exalted) does so in order to draw closer to Allah and in compliance with His command, even if his relatives cut him off. But if he upholds ties only when they do so, this is like paying off a debt. Hence [the Prophet (blessings and peace of Allah be upon him)] said: “The best charity is that given to a relative who is hostile. That is because spending on a beloved relative may be mixed with personal inclinations or whims and desires, whereas in the case of spending on a hostile relative, there is no element of whims and desires.
End quote from Kashf al-Mushkil (4/120-121).
Your showing kindness to her will, by Allah’s leave, be the best punishment for her, for it is a means of being granted victory by Allah for her transgression against you.
It was narrated from Abu Hurayrah that a man said: O Messenger of Allah, I have relatives with whom I try to keep in touch, but they cut me off. I treat them well, but they abuse me. I am patient and kind towards them, but they insult me.
He said: “If you are as you say, then it is as if you are putting hot ashes in their mouths. Allah will continue to support you as long as you continue to do that.”
Narrated by Muslim (2558).
An-Nawawi (may Allah have mercy on him) said:
“you are putting hot ashes” – hot ashes are like hot sand…
What is meant is that it is as if you are feeding them hot sand. Thus he likens the pain that they will feel to the pain felt by the one who eats hot sand, and the one who treats the other with kindness will not be affected and there will be no blame on him; rather they will incur great sin for cutting off ties of kinship with him and mistreating him…
End quote from Sharh Saheeh Muslim (16/118).
The words “Allah will continue to support you as long as you continue to do that” – What this means is that Allah (may He be exalted) will help you to bear their harshness with patience and to have a good attitude towards them, and He will cause you to prevail over them in this world and the hereafter, so long as you continue to deal with them in the manner mentioned.
End quote from al-Mufhim (6/529).
You should constantly offer supplication (du‘aa’) and continue to treat your sister well and show patience towards her, for this patience will raise you in status and dispel enmity by Allah’s leave.
Allah (may He be exalted) says (interpretation of the meaning):
“And not equal are the good deed and the bad. Repel [evil] by that [deed] which is better; and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend
But none is granted it except those who are patient, and none is granted it except one having a great portion [of good]”
If you are not able to attain this high level of generosity and responding to your sister’s mistreatment with kindness, seeking therby the pleasure of the Lord of the Worlds, and hoping to ward off her mistreatment of you, and keeping in touch with her will affect you adversely and be harmful for you, then there is no blame on you – in sha Allah – if you cut off ties with her as much as will ward off harm from you and protect you from the harm that she is doing to you.
Ibn ‘Abd al-Barr (may Allah have mercy on him) said:
The scholars are unanimously agreed that it is not permissible for a Muslim to forsake his brother for more than three days, unless he fears that speaking to him and upholding ties with him may undermine his religious commitment or lead to some harm affecting his religious or worldly affairs. If that is the case, then he is allowed to avoid him and keep away from him, and perhaps cutting off ties and shunning him in a good manner is better than mixing with him if that will result in harm.
As the poet said:
If mixing with others will only mean baring your teeth at one another, then keeping a distance in a peaceful manner is best for both parties.
End quote from al-Tamheed (6/127).
See also the answer to question no. 143596 .
And Allah knows best.