Weddingbee boards

So I found out my younger isster is planning on bringing her new boyfriend to my 9 year olds basketball game tomorrow. This doesn't suprise me because she always brings a guy with her everywhere.

The problem is this guy is a new level of loser and not really a person I want around my kids. Not only is he 28 (she is 21), with no job, many drug and alcohol violations, but a few weeks a go she took a disorderly conduct/noise complaint at his apartment so he wouldn't have a probabtion violation. He had no problem with her getting in trouble for it and I am pretty sure asked her to do.

This guy was in my older sisters class in high school. A few years ago he was in a car accident and they were all drinking. One of the other passenges died and him and another friend moved their dead friend's body into the driver's seat so the driver wouldn't get in trouble. This is the most messed up thing pretty much ever.

Anyways my main question is how should I tell her I don't want him around my kids without putting her on the defense?