Motzei Shabbos is because that is when neshamos go back to gehinom (RMA YD 376:4) daily mariv because midas hadin is more powerful at night (KSA)
Regarding siblings during shiva there is no obligation to make extra minyanim. I just saw this yesterday but don't remember exactly where. I will try to check this up.
I specifically remember as kid not only my Dad davening Motzei Shabbos before a yahrzeit (even if it wasn't bo bayom), but that our shul would split into multiple minyanim if more than one person had a yahrzeit that upcoming week. (regardless of how many chiyuvim there may have been).
It would be for every tefila. The question is if he needs to change his schedule to get every tefila. Most important is mariv motzei shabbos then weekday mariv. There is definitely no obligation to change shuls for it (besides that he doesn't have kedima for the amud in a shul other than his regular one and would only get it if there is no one else with any level of chiyuv at all-even during shiva). It is important to note that the bigger zchus for the niftar is davining for the amud over kaddish, which was only instituted for children and others who can't daven for the amud.
I've also been told that learning (specifically mishnayos, because mishna and nehsama are anagrams) are also more important than kaddish.
As for Shiva and siblings - I am assuming that leading tefila is a zchus of the deceased. Therefore if one brother or the other has the amud, their relative is getting proper zchusim. (my assumption).