Translation:My uncle advised that I inform him about my sister.
Why "Mia onklo konsilis, ke mi inforu lin" instead of "Mia onklo konsilis, ke me informu sin"? Does this sentence clearly me that we should inform "him", where "him" is not my uncle?
No, it's not correct. "Sin" is used only with li/ŝi/ĝi/ili and refers to a subject (in this case "mi" is a subject of the clause). You can say: "Mia onklo konsilis, ke mi informu la/tiun onklon" if context is not clear.
There are two clauses seperated by a comma. The first: Mia onklo konsilis, The second: ke mi informu lin pri mia fratino. Each of them has its own subject. In the first clause: "mia onklo", in the second one: "mi"
If the sentence was:
"Li konsilis, ke mi informu ___ pri ..."
Would we use lin or sin?
"lin" in the second clause likely refers to a fourth person.
(the speaker the uncle the listener the person who should be informed)
Why "inform" ? Why do not "My uncle advised that I informed him about my sister." ?
Yea I am also perplexed about this. Grammatically should be "informed", right?
Why is inform in the imperative here? The notes say:
The -u ending is used when ordering / inviting someone else to do something -- or when telling or suggesting to ourselves what to do!
In this case my uncle is advising me, not ordering, requiring nor inviting me. Shouldn't we be using the infinitive here?
Are the notes wrong, or incomplete, or is the sentence wrong?
There is one other possibility, and that is that we are using doublespeak, where to advise implies a threat. If this were the case, then I certainly wouldn't want an uncle like that!
It's one of the functions of the -u form but not the only one. If you look at Tips Notes more closely, you can read:
THE -U ENDING IN SUBORDINATE PHRASES
We also use the -u ending in subordinate phrases (clauses) starting with ke, when the verb in the preceding, main part of the sentence expresses a want, desire, demand or preference:
But it is not expressing a want, a desire, a demand or a preference. It is about advice, which is none of those things!
Isn't it preference? Anyway, it says what something should be/what should be done not what it is.
Citing form Wikipedia:
The sentence containing the deontic modal generally indicates some action that would change the world so that it becomes closer to the standard or ideal.
The only advice worth having is that which is given disinterestedly. I have certainly given advice suggesting that the advisee do something that I would prefer that he didn't do. So, no, it is not a preference. It might well be deontic modality. (Thanks for that.)
Anyway, I'll just have to get used to it. Advice is a suggestion and the notes specifically say "... suggesting to ourselves what to do ...". I should, therefore, have worked out that a suggestion to other people also takes the imperative.
My interpretation is that -u is appropriately used whenever there is a persuasive intent (and as such, the verb "advise" would clearly be included).