Persons is used in a more formal context like legal document. People is used in conversations. This is generally speaking of course.

I could be wrong here but it may be that your sister is not laughing at your use of the word since it's actually correct. it could be she's laughing at your generalization of "those people" as that can sometimes be seen as condescending or derogatory.

For example, one time I was talking about how my sister and her family/household travel often, saying, “Those people travel a lot,” and the person repeated those people and gave a slight laugh. I’m wondering if I may be giving some sort of unintentional implied message when I use that word.

I joke around friends who accidentally phrase things in ways that could sound like bigotry/racism if taken out of context, and it sounds like that it what the person was laughing about.

'Those people', when used while judging or looking down on somebody is a common way for bigots and racists to avoid using slurs around non-bigots/racists. Something like "The park was a lot more fun before those people showed up." while nodding in the direction of some people with darker skin. Or saying that 'those people' are doing something unacceptable.

It isn't a people vs persons thing, it is specifically the phrase 'those people'.

"People" refers to a group of humans as a unit. "Persons" also refers to a group, but puts emphasis on the individuality of the humans in the group.

https://www.thesaurus.com/e/grammar/persons-vs-people-vs-peoples/

It’s not “people vs persons” but “those people vs they”.

Conversationally, “those/these” distances you from the group you are talking about, which is humorously weird when it’s your family you’re talking about.

It’s not the meaning of the words, but habitual (and often fleeting) attribution around them that tripped you up.

PS: “People” are uncountable, “persons” are countable. That’s basically the whole difference between the two plurals. Although it’s rapidly disappearing, as “ten people” won’t raise a single eyebrow in a conversation.

"People" is a generic term for more than one person.

"Persons" denotes a singular distinct grouping of people. Ie, Native American persons.

Not part of the question, but "peoples" is used for a plurality of distinct persons. Ie, "this had great impact on the various peoples of North America" would be a sentence to lead into a discussion on how an event had varying impacts on each unique cultural group in North America. This is largely only used in academics, specifically anthropology and sometimes sociology, but understand this use helps clear up the reason for the distinction between "people" and "persons".

I think it's like the difference between "fish" and "fishes."

"Fish" refers to a group of the same type of fish, but if there are several groups of many types of fish, you would say "fishes."

My understanding is that "persons" refers to people in discrete groups, where what separates those groups is pertinent to the topic being discussed.

For example - 'indigenous persons' refers collectively to indigenous people, but acknowledges that there are separate subgroups of indigenous people within that. You could equally say "indigenous people" and it would be correct but you lose that nuance by not acknowledging that there are internal divisions within the group you are referring to.

Not saying that's the dictionary definition, but that's how it's generally used in my field.

Persons is a set of individuals while people generally implies a group. "Those people" as a phrase carries negative connotations because of its use by racists pretending to be not quite as racist.

You can use them interchangeably as long as you're trying to imply the person's you are referring to are linked or similar in some way.

You'd get even weirder looks if you said "those persons travel a lot", while also sounding like someone who doesn't really speak the language.

"Those people" can be a racist or classist dog whistle, but isn't always, and also there isn't really an alternative. Say what you're going to say, and don't worry too much about it. The people who would misinterpret it to fit an agenda are probably going to do so regardless of what words you use.

Ok, now I'm not a linguist and also not English, but in my opinion there's a difference. "People" is a broad undefined group. "Persons" is a more defined group.

Adding "those" in front also alienates them further. So by saying "those people", you are distancing yourself from them, despite them being your family. That is the amusing part for the person you talked to. You mildly insulted your family.

In a similar vain, "some people say" all the stuff that you won't to be heard saying yourself, even if it's your own opinion.

Or in some places (this might be more local) using "you"or even "one" instead of "me" when answering personal questions. For instance if someone asks: "How does it make you feel?" and answered: "It makes you feel sad" then the person answering it is distancing themselves from their own answer by literally answering on behalf on some unknown "you" when they should be using "me". Using "people" is sort of the same just on behalf of someone else.

Good question, and I'm not sure of the actual, lexicographic answer.

All I can say is there's typically an implicit negative connotation when using the form "those people" regardless of intent. Usually it's used that way when stereotyping or otherwise making a blanket statement about a group, so even benign uses of the phrase tend to sound hostile.

My guess is that "those persons" sounds more specific.

It may helpful to think of this in terms of human rights; some rights apply to individuals so even though there may a group of them we'd refer to them as persons e.g. displaced persons. Some rights are held collectively and we would refer to them as a people e.g. Indigenous People of the Amazon.

eta: "Those people" and "you people" are both seen as racist dogwhistles. Your sister was probably laughing because you didn't intend or get the subtext of your phrasing.

You people, and I mean you people

sometimes we gotta rework our phraseology

Sometimes people will say "That person's name!" or "Those group of people!" in anger. "That Donald Trump! How dare he claim immigrants are eating pets?" to give you a current example.

When spoken of a family member or mutual acquaintance with a chuckle, it means more like "That person has some strange quirk but what can you do? We still love him."

For example, you might hear "That dog! Always chasing his own tail." So I think this is likely what you were getting from that conversation? It's certainly not a criticism of your use of the word "people".

Unknown people can be called persons unknown…