What is the way to overcome feelings of hatred towards someone who wronged me, even after the problems have ended, and is it a sin to ignore a female friend who mistreated me?

The best way to help you overcome hatred for your mother-in-law and sister-in-law is to remember the reward for forgiving those who wrong you, as Allah Almighty says: {And let them pardon and overlook. Would you not like that Allah should forgive you? And Allah is Forgiving and Merciful.} And the Prophet (peace be upon him) said: "And Allah increases a servant in honor by nothing but forgiveness." Also, consider the harm of occupying your heart with this feeling, and what it might lead to in terms of backbiting, envy, and slander. As for limiting your relationship with your friend to merely greeting her due to the impossibility of reconciliation, there is no sin upon you for that.