I am the only one who can say kaddish for an older brother (unless I asked somebody who is a non-relative) and there is no issue about my parents’ consent as they are both long since passed away. There are two issues I am being confronted with. The first has to do with to what extent I should go to get to shul motzei Shabbos to say kaddish. I live far from Shul and going back for Maariv will disrupt the family greatly, leaving me only half an hour at home after Mussaf before a difficult journey back to shul.
The second question, which may impact the first question, is to what extent I should go to lead the services (shaleach tzibur) in light of the fact that there is another person in shul who is saying kaddish for his father who passed away about 5 months ago.
You should not delay the minyan for Maariv. This is a tircha de-tzibura and it is not appropriate to do so for any purpose other than some urgent communal matter.
As for the other options you mention, I don’t think coming to Maariv will justify putting out your family and yourself to the degree you mention. It is true that a yahrzeit for a brother is not a true Chiyuv. The reason for this is that the principle of honoring a yahrzeit in general, and in particular of davening as the shaliach tzibbur and saying Kaddish in particular, is that the merits of a son reflect on the father. This doesn’t apply, at least not directly, to a brother — though for a firstborn brother there is an element of Kavod that some authorities mention.
Therefore, although it is proper and correct to say Kaddish on your brother’s behalf, and if possible daven from the amud (also, if possible, fast for the day), I do not think you need to put yourself and your family out to the degree you mention for this purpose. Say Kaddish and daven from the amud for Shacharis if possible (you don’t defer somebody who is a chiyuv, but if he agrees you can definitely do so), and learn some Mishnayos on your brother’s behalf.
For the comparison between the obligation concerning parents and that of a brother, see Be’er Eliyahu, Yoreh De’ah 212; Be’Tzeil Ha-Chochman Vol. 4, no. 22, sec. 4.
The Mateh Efraim (Kaddish 3:14) mentions the yahrzeit of a grandfather, adding this to parents – but this does not extend to a brother.
Ma’avar Yabok (Sifsei Renanim 21) notes that fasting on a deceased’s behalf is beneficial for all.