Is it permissible for an elder sister to ostracize her brother's immodestly-dressed wife, and to completely cease all interaction with her, neither meeting nor communicating, especially after having previously tried, without success, to accept her?

You have given your brother good advice. This advice should continue by him raising his wife in goodness, teaching her her religion, and obliging her to wear the hijab. He should be reminded of his responsibility for her before Allah, as Allah the Almighty says: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones." It is permissible to abandon a girl if she is among those who commit sins and it is believed that abandonment will deter her. However, do not rush to abandon her. It is better to be gentle with her and advise her, for perhaps advice will benefit her and she will be guided. There is great reward in that.