Is there any difference between "a few relatives" and "a few relations"?

Sometimes two words can be perfectly suitable synonyms, but we tend to stick with more familiar phrasings when they are put into familiar expressions. Here's one example:

^{Ngram: friends and enemies (blue), friends and foes (red), friends and adversaries (green)}

If you add pals and adversaries to that search, the Ngram won't return any results, and Google only finds a scant 37 instances of pals and adversaries on the web. There's no reason a less-trite pals and adversaries couldn't be used, except some authors may prefer a more familiar expression.

The same can go for the ordering of words in familiar expressions:

^{Ngram: ladies and gentlemen (blue), gentlemen and ladies (red)}

Obviously, there's no difference in meaning between ladies and gentlemen and gentlemen and ladies, but one sounds more familiar, which makes the other sound more jarring in some contexts.

You're right to point out that friends and relations seems more rare in American English; I can't recall hearing that wording, but friends and relatives seems as common and well-worn as ladies and gentlemen.

With all that in mind, here's how I would rule:

Is there any difference in usage? No – although one may be more common than the other, especially geographically.

Are both terms interchangeable? Insofar as I can tell, yes. If you used one wording instead of the other, I don't think anyone would feel slighted, and I don't imagine you'd be written out of any wills.

Are relatives and relations, perhaps, truly synonymous when referring to family members? "Truly synonymous" is a loaded term; I would feel more comfortable saying that they are "largely synonymous." There may be certain contexts where one word would be more fitting than the other, but I'm having trouble thinking of one off the top of my head.

Is there any difference between “a few relatives” and “a few relations”? I suppose not, although, due to additional definitions of each word, a few relations could be construed to mean “a few of my relationships” instead of “a few of my relatives,” if there's not enough context to disambiguate, as in: a few relations have been stressed lately.

My speculation is that "relations" now carries a rather negative connotation. The most common use of "relations" that I can think of is of a sexual nature. Usually, trying to sound more tactful, media and other outlets will basically use this type of terminology over anything more direct.

I personally would not feel comfortable using this word for this reason alone.

In the Law of Wills, "relatives" are legitimate and "relations" are related by blood whether lacking legitimacy or not:

Source

The popular meaning of the word "relatives" or "relations" is that of all persons within any degree whatever of consanguinity or affinity. But when the word "relations" is used in a will to denote a class of beneficiaries, it is settled that the law imposes a technical meaning and not the popular one. The primary meaning of "relatives" or "relations" is, such persons as would take under the statutes of descent and distribution if testator had died intestate. ...

The primary meaning of "relatives," of course, imports legitimacy. But this meaning may be extended by the context. Thus certain persons who were related to testator by blood, but were illegitimates, were referred to by him in his will as "cousins." It was held that these persons could take under a bequest to testator's "relatives hereinbefore named," especially since the bequest to these "relatives" would be a bequest to one person only, if the illegitimate relations were excluded.

The only example I can find where the two are not interchangeable is in the expression no relation". When two people have the same surname but there is no family tie, the words no relation are often inserted e.g. G. Smith, K. Smith (no relation). It is used to negate, to separate, not to link.

In French, the word relations exists, but it means contacts in a general sense. Ce sont des cousins appears to be the best translation of "these are my relatives" even when they're not cousins. Might this partly explain the lesser intimacy perceived by some in the word relations? And the propinquity of the term sexual relations might alienate others. Could the word relationship play a role in distancing relations from us. A relationship seems less solid and enduring than a marriage, viz Facebook status "in a relationship".

All in all, of the two words, relatives seems to me to have the more clearly defined contours and, as such, less likely to be misunderstood.