Why is my sister so negative about men?

So from the moment I met my partner. Instead of I am happy for you it was I am the only single one now. I found it super selfish. I have watched her in relationships whilst being alone as the younger sibling.

It never bothered me. I also made effort with her partners. She never has with mine. She is a single mom of three with two baby fathers. I have now been with my partner for 4 years and we have a son and live together.

She always says sarcastic things about men like , they all cheat. I said if my partner wasn’t happy he would just end it. She had a domestic violence relationship ten years ago. Then she had a good one. She left her relationship. An after me and my partner had a bad argument a month ago.

She tells me to leave. Said he’s not my life partner he was just part of my journey. Said he doesn’t care about you or support you. He controls you and says men like that are this and that. It felt like an attack as the past few years she hasn’t been in my life or been around my relationship to know what it’s like.
She says once you leave you will feel so relieved. Me and my partner have talked of marriage and we did once get engaged, however that was met with negativity so we decided to wait. She phoned after receive the news saying something to tell me.

Then we ended call she rang back saying I am happy for ya. It felt very false to me and not genuine. She dated a married separated guy he left her for his wife. Her most recent boyfriend they lasted 6 months as he wanted more kids she doesn’t. She says stuff like don’t hold on too long. An that my son won’t know he’s not around. We are a loving family. So it hurt she said that.

She tried to say he’s cheating on me and I am naive because I say he’s not that person. She only wants to be a sister when something bad or minor happens. Like an argument. Otherwise she disappears she messaged me recently asking how I am and I said I am good and got met with silence?
She’s nearly 30 and I find it immature.
Why is my sister so negative about men?
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