Maybe obvious, but uh: don’t fucking do this! I get it, I'm an older brother, I would wanna go ballistic too. But the fact that these guys skipped over "notify the police" and went straight to "seek out in-person confrontation" is insane! Stalkers are not people in their right mind. Staking out and planning to “citizen’s arrest”(???) someone who has repeatedly visited late at night to threaten and harass is NOT SAFE…
However it is worth noting that the sister had been to the police quite a bit by the sounds of it.
"Now it was time to call 999. My sister had already had a couple of police visits about the previous incidents, so they knew there was a problem linked to her flat and came round pretty quickly."