Let me ask you a question.Physically ill, because of that comment.
Look, not all of them are bad, and it took my sisters fiancée to make me see that. He is a great guy, and he loves and respects my sister a lot.
People are people at the end of the day, regardless of lifestyle.
Does his group of travellers, not just him, respect your sister?
Add to that, and this is more important.. does his group respect the law of the land and the local inhabitants where they decide to camp? By this I mean do they teach their children decent behavior, control their dogs, keep the camp site clean, follow the instructions of local authorities and pay taxes, etc?
If not, they are criminals.. being a nice guy to your face doesn't mean any of these things are true.
Edit: To those outside the areas where these groups are.. let me explain. I grew up in rural England, we had a designated traveller camp down the road from us.. but newcomers would feel free to set up their trailers on literally any space of open ground, including playgrounds, roadsides, or in the street in the middle of the village. Anywhere they camp almost instantly went to hell, they let their dogs run wild, scavenging from rubbish bags. Rather than sending their children to school, they send them to the local shops where they would shoplift in huge groups, threatening anyone who asked them to leave or called the police. They dump their toilets down the drains in the street, and leave garbage strewn around everywhere. Many of those who decide to live in the permanent camping areas provided, at the tax payers expense, continue this behaviour.