What does Jack Sparrow mean with this word of advice?

The point of the advice is that a brother might tolerate a man courting one of his sisters, but he will consider a suitor to multiple sisters to be a cad and will therefore oppose the suitor, so in that case the pirate is suggesting killing the brother before he becomes an obstacle.

The word "savvy" is a corruption of the Spanish phrase sabe usted which means "do you know?"

Tyler Durden's answer explains, I believe, the full meaning of all the words. And he explains the scenario in which all the words make sense: killing the brother so that he does not attack you when you are courting one of his sisters while also pursuing another.

Why Jack said "brunette" is unexplained in that answer.

I believe the "hidden" meaning is that when Jack courted a brunette in the past, he also pursued her sister. Resulting in the trouble with the brother. The reason for the word brunette is meant to suggest that he is referring to a specific experience. Otherwise the addition of brunette over any other hair type is irrelevant.

The hidden meaning is a suggestion of Jack's history inserted into the advice.

But. I wouldn't call it the "hidden meaning". I would call it the joke.

As a metaphor, I think what Captain Jack is saying is:
"Don't do stupid things, but if you have to do them, take precautions."

It's a bad idea to date a girl and her sister at the same time, but if you're going to do it, the biggest threat is their brother. So kill him for safety.

I don't think there's anything much more to it than that, though it's suspiciously specific enough (regarding Brunettes and the scenario) that it implies this is something that's happened to him personally.

The phrase doesn't seem to reference anyone specifically in Pirates of the Caribbean's canon, as it is to date. It just seems to be a funny anecdote

However, I do think Jack brings this up in conversation for two indirect reasons.

1. He is saying this to Henry as warning, recognizing that Herny seems to like Carina, who happens to be a brunette. Jack had been previous arguing with her during the St. Martin bank robbery escape sequences, where he uses her to make his escape.
2. Ironically the bank robbery scene opens with a funny reveal of Jack being inside the bank vault in company of the Mayor's wife, Francis. She too is also brunette.

So I think Jack made a little a jump here that makes it all a little more amusing.

That doesn't mean that the line was not also put here to eventually reference someone in particular, as even the line from Curse of the Black Pearl, "Clearly, you've never been to Singapore." didn't make much sense until the third film, At World's End.

As food for thought, there are a few brunette characters from Jack's childhood, but none of them had a sister and a brother.

There is also Angelic introduced as a scornful love interest to Jack Sparrow in On Stranger Tides. The two clearly have history, but not much is know about Angelica's past or if she actually has siblings, given the convent situation? There are some loose ends in that film that could play into future installments, including that the final scene features her stranded on an Island with the Jack Voodoo doll in hand. We have no way yet of knowing if that at all played into the plots of Dead Men Tell No Tales or what happened to Angelica.

Yes, I would say that there is a "hidden" message, in the sense that Jack's language is euphemistic.

• When Jack says "courting" he means "trying to have sex with"—not the standard meaning for "courting" in seventeenth/eighteenth century English, where it generally meant "trying to convince (someone) to marry you" or just "trying to gain favor with (someone)".*
• Likewise, when he says "if you cannot avoid the charm of the sister" he means "if you go ahead and have sex with the sister", rather than just "if you find the sister charming".

In these circumstances, a brother could be expected to take offense on behalf of his sisters' honor. The traditional response would have been for the brother to challenge Jack to a duel and/or force Jack to marry one of the sisters, presumably at the point of a sword or gun.

• So when Jack says "kill the brother" he is either suggesting winning the duel or preemptively murdering the brother to circumvent the whole rigamarole, depending on just how dishonorable you think Jack is.

• _{Though we know that Jack was not above falsely promising marriage in his pursuit of women.}

This is just a way to show that Jack is a man of "relaxed morals", by telling how once (at the very minimum) he dropped his girl to go with her sister.

The point here is that Jack is such a womanizer (or, if you like, amoral) that:

1. his only worries and regrets about the whole issue are the practical implications of their brother taking offense and that and attacking Jack. Jack does not worry if what he did was right or wrong, if he hurt the women's feelings,...

2. he assumes that everyone is like him, so he gives advice to Henry so Henry can avoid the problem if Henry wanted to change Carina for her sister (and then again, the advice is not "don't do it" but "do it but make sure that it does not kick back at you").

Additionally, it also shows Jack as a violent man who is not afraid to use violence to get what he wants.

All of this is explained in a short, extravagant advice as it is more "family friendly" that Jack Sparrow just telling "I once had a girlfriend and then went with her sister, I later killed their brother".