Future sister inlaw is a potential psycho?! 😵🙈

What does it tell you about a person who has said to future mom inlaw some really nasty things face to face? My brother got engaged last december, to what we at first felt like a very sweet girl and we are super happy for the both of them and really excited to welcome her to the family. But a month later they couldnt iron out some major personal issues, theyve been arguing alot.. Recently she (fiance) sort of lashed out at my mum and said something along the lines of "if your son continues to provoke me, i can be nasty too!", she also have said "i gave up a rich guy for your son.. i could have had it easy but i chose to stick around with him (my brother)". these comments were said to my mum's face.. and my mum was so hurt but despite that she never said anything back because she knows how smitten my brother is with her.. The fiance is 29 years old.. i honestly feel like she's taking my brother for granted and as though he's replaceable.. 😔 im sorry for the long rant.. this is whats been keeping me up the past few weeks.. i hope someone can share any similar experiences dealing with.. i dont even know what im dealing with! 😔