Tresspassing & Harassment

Hi, My sister is married and setup her own home long time back. She has 2 grown-up kids who are are now adults. Her husband recently passed away. For more than 2 decades, she has not been behaving well with my mother whom she thinks did some injustice to her. Consequently, she has regularly harassed her causing her severe distress. My mom who is a senior citizen has tolerated this despite my advise to take legal recourse and police assistance for the reason that she is her daughter. However, recently, this has become even worse. She has permanently set herself up in our 3 bedroom flat wherein my my mom and 2 of us brothers (along with spouses live together with children). My mom is a soft person and she did not stop her. She told her that this is not proper but my sister is not listening and creating a scene thereby causing disrepute to our family as well. As info, the flat is owned by mom alone. Please advise what can be done from a legal perspective realistically. My understanding is that no one including a family member can force themselves in her flat without her permission and this typically amounts to trespassing and harassment. From an evidence perspective, she also used to call at least 20 - 50 times every day as part of harassment before she setup permanently in our flat. Thanks in advance for your advice.