Answer

1. It seems to me that yes. The proof is from the Talmud (Eruvin?) cited by Netivot HaMishpat, section 234, regarding one who sees his rabbi committing a transgression unintentionally—that he is obligated to point it out to him (except that in rabbinic matters one points it out only after the act).
2. From that same Talmudic passage it is proven that this also applies to rabbinic prohibitions. Except that regarding rabbinic matters, the accepted halakhic rule is: it is preferable that they remain inadvertent sinners rather than become deliberate sinners (as brought by the Rema in the laws of Yom Kippur regarding women on the eve of Yom Kippur).
3. As stated, there is also an obligation under the law of rebuke. As for "love your neighbor," it seems that this depends on those same words of Netivot HaMishpat, since according to his view, an unintentional violation of a rabbinic prohibition is not considered a transgression at all. Therefore, you are not preventing him from a prohibition, and so this is not a matter of "love your neighbor." The obligation to rebuke him is in order to prevent transgressions (so to speak, for the sake of the Holy One, blessed be He, or the world), and not to prevent harm to him (that is, it is not an obligation for his sake).
Of course, if there is an obligation, then there is no difference based on closeness. Obligations, by their nature, are universal (that is, toward all Jews). Only in matters beyond obligation (or when it is impossible to do it for everyone and one must choose one person—such as "the poor of your own city take precedence") is there room to distinguish based on closeness.
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Questioner:
I encounter on a daily basis many transgressions that people commit, knowingly or unknowingly, Torah-level and rabbinic-level, and I notice that the overwhelming majority of people do not rebuke one another at all. If I were to act strictly by the book, I would have to spend most of my time in rebuking conversations whenever I am in the synagogue. I am aware of the rule "it is preferable that they remain inadvertent sinners rather than become deliberate sinners," but it seems to me that in quite a few cases people actually would accept the rebuke. If so, how should one properly conduct oneself with regard to the commandment of rebuke?
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Rabbi:
The halakhic decisors write that the rule "it is preferable that they remain inadvertent sinners" was said only about rabbinic laws, not Torah-level transgressions (the source is Beitzah 30a). But it seems to me that the underlying assumption is that the purpose of rebuke is to correct, and if it will not lead to correction but only to quarrels, then what is the point? In their times, however, the assumption was that if you pointed out a Torah prohibition to someone, he would usually acknowledge it and correct his ways, whereas with a rabbinic prohibition not necessarily. This is similar to the law of public proclamation regarding those disqualified from testimony because of rabbinic prohibitions, where the assumption is that people are not aware of rabbinic prohibitions, unlike Torah prohibitions. Still, where the rebuke will help, there is certainly an obligation to rebuke.
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Questioner:
In the synagogue where I pray there are many people who are unaware of rabbinic prohibitions (such as the prohibition against walking in front of someone praying, or preparing on the Sabbath for a weekday). Most likely, if I sat down with them and explained the prohibition and showed them sources, they would be convinced. The problem is that practically speaking, this is very hard to implement. I am exposed to masses of such transgressions in the synagogue, and if I rebuke each person, I will not be able to pray. Also, I do not notice others rebuking— even rabbis when they see others violating such prohibitions— and from that I learn: "go out and see what the people actually do." In addition, some people (especially older ones) would not look kindly on a young fellow rebuking them (although it is still possible they would listen, but they might also bear a grudge).
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Rabbi:
There are situations in which one does not rebuke because one estimates that it will not be accepted, or because one fears that it will create a commotion and discomfort. And that is legitimate (see below). There are situations in which one does not rebuke because there are halakhic approaches that do not forbid the act (that is, it is not an unequivocal prohibition). Although within the parameters of "you shall surely rebuke" it is written that one should rebuke even up to being struck, the original parameters of the commandment do not apply today, and I will explain.
It seems to me that the basic halakhic explanation for refraining from rebuke is that already in the Talmud itself it is said (Arakhin 16b) that in our generation there is no one who knows how to rebuke and no one who knows how to receive rebuke (just as you yourself describe). Therefore people generally refrain from rebuke, and I think rightly so. There is an atmosphere in which people do not accept comments from others, and it angers them (even if they keep quiet and do not say anything), and so the consequences of rebuke are not always positive. As a general rule, in my opinion it is better not to rebuke unless it is a friend of yours and you do it gently, in a neutral setting, and politely explain that you do not intend to offend, etc. That is not so practical in everyday life and in the synagogue. In sum: there is room to make an effort to rebuke, but one definitely needs to think carefully whether it is appropriate to do so. In many cases this is really just venting frustration at someone who bothers me or angers me with his disregard for Jewish law, and not an act whose purpose is to correct.