Help sisye keeps harassing we were meant to get engaged

So on July the 21st my sister called up for a smoke and my partner took it around there an dropped it off to her

he came back an hour and a half later I asked what he was doing he said she had him back and forth to the survo station to get stuff I believed him I got a phone call from her 3 days later and she’s like sis can you please come around I need to talk to you and I was like okay and she’s like bring him has well but he didn’t want to come

So I went around there SO she said he went out to feed the dog then when he came in she chopped a mix and asked him if he wanted a cone so she said he had 3 cones of weed and she said she asked if he can help grab a suitcase on the top cupboard not sure how this came by but she said he bumped in into her back side with his front side I don’t know how if she was standing there or whatever she should of moved if she asked it to grab something I don’t know how it happened and his like sorry sis I’m leaving then his like can you walk me to the front door course she had to lock it becourse if the kids they were asleep anyways has he stepped down the step she said he went off balance and fell back and try to grab something for support but she said his hands went between her legs has if she was putting a pad on we walked back around together to confront him he said he didn’t mean it how she’s thinking or trying to take she was putting it this is my 30 year old sister she was drinking and also popping pills

Update she keeps calling message I asked her what’s her problem course of all this shit I’m over it she comes around while I’m asleep for the 3rd 4th time she said to my 2nd eldest sister why can’t he just tell her the truth I said to her if he means that much to you to keep arguing over him have him why would I want someone my sister is crushing over mind you she has a kid to his step cousin but she thinks they actually related but ain’t

She 30 has 5 kids an a partner

He won’t leave I’m over feeling like I’m the bad person how do I even deal with this