Should I leave....

I been living with my girlfriend and her family but around 3 weeks now her ex started living in a text out the back garden.

I'm in a tough boat as I don't want to keep her ex there but when I say anything her mum and herself says there's nothing to worry about or she won't get back with her ex.

But I'm scared been through it before for my old relationship what should I do just stay and bite my tongue or should I go

We been together for year and nearly 9 months should I just leave and give up the relationship...