Yerusha: vertical only or also lateral

This article is an analysis of the mechanics of yerusha, distilled to its purest form, showing how differing abstract conceptions thereof have decidedly real world consequences 

The parasha (in this case literally) of nachalos (Bamidbar 27:6-11) prescribes that upon death an estate first passes to ones children (male, then female), lacking that (or any descendants) it proceeds upward to dad (see Ramban to verse 9), and if hes no longer around it goes to his children ie the siblings of the dear departed (including descendants), with this pattern continuing on ever upward. 

At its core, biblical inheritance seems to follow a simple two step vertical formula: first to ones progeny (with the male having precedence) and then to ones (male) progenitor. This formula repeats until finding a living yoresh. 

According to this a person bequeaths in both vertical directions: downward to ones children and upward to ones father. In other words, yerusha is limited to ones immediate vertical relatives. 

Ramban (pasuk 9) seems to concur with this: ועוד, כי הירושה היא בשלשלת הזרע ביוצאי חלציו לא בצדדין. אם כן: ונתתם את נחלתו לאחיו – משמוש נחלה שהאב יורש בקבר, וממנו תבא לאחים, ditto for Ralbag (ibid) ואם אין לו בת – חזרה הנחלה לאביו, ולזה מחלקים אותה אֶחָיו מאביו ביניהם.

There is a halacha (BT BB 130b) that one can manipulate the yerusha increasing and decreasing the default portions as one sees fit (amongst actual yorshim eg sons, or daughters when there are no sons or descendants).

Rambam illustrates it as follows (hilchos nachalos 6:2) הָיוּ לוֹ יוֹרְשִׁין רַבִּים כְּגוֹן בָּנִים רַבִּים אוֹ אַחִים אוֹ בָּנוֹת וְאָמַר כְּשֶׁהוּא שְׁכִיב מֵרַע פְּלוֹנִי אָחִי יִירָשֵׁנִי מִכְּלַל אֶחַי אוֹ בִּתִּי פְּלוֹנִית תִּירָשֵׁנִי מִכְּלַל בְּנוֹתַי דְּבָרָיו קַיָּמִין apparently including ones brothers in this halacha. 

Now, if one only directly bequeaths to ones father, and it's the father (having inherited in the grave) that in turn bequeaths to his children (the decedents brothers) then the dear departed should not be able to have a say on how it is divvied up, as his estate was already inherited by his father and it is his father's estate that is now being inherited by his (dad's) other sons. 

Tur (CM 281:2) does not make mention of the deceased's brothers when codifying this ruling, ostensibly signaling his disagreement with this enlargement. 

SA (CM 281:1) quotes Rambam's language verbatim. Ketzos (with Nesivos approving) addresses this issue arguing that he is nevertheless legally viewed as the effective morish and can therefore take advantage of this provision. Based on that they posit that this would not be limited to ones brothers but would also apply to grandsons (Ketzos) as well as to nephews (Nesivos). 

Presumably, taken to its logical conclusion, this would hold true wherever a yerusha ultimately lands up (so long as there are two or more yorshim), maximally expanding the scope of this halacha. 

R. Akiva Eiger takes a different approach, proving from the fact that MT and SA include brothers in this law that Rambam and R. Karo are of the opinion that an inheritance does not initially trace it's way to ones father and only from there on to ones brothers (mishmush bkever), but rather, if ones father is already deceased, proceeds directly to ones brothers, entirely circumventing the dead father. 

In other words, a nachalah, after seeking out a vertical nochel, redirects and begins to move laterally to ones brothers.

According to this understanding, biblical inheritance, at its core, is comprised of three steps: downward, upward and sideways. 

Taking this approach would see this law limited to what Rambam spells out, to wit, children and brothers, as all other (practical) yorshim are not directly inheriting the deceased. 

There are a few halachos right at the beginning of hilchos rotzeach (1:2-3) that read as follows: מִצְוָה בְּיַד גּוֹאֵל הַדָּם [לַהֲרֹג אֶת הָרוֹצֵחַ] שֶׁנֶּאֱמַר (במדבר ל"ה:י"ט) גֹּאֵל הַדָּם הוּא יָמִית אֶת הָרֹצֵחַ. וְכָל הָרָאוּי לִירֻשָּׁה הוּא גּוֹאֵל הַדָּם. לֹא רָצָה גּוֹאֵל הַדָּם אוֹ שֶׁלֹּא הָיָה יָכוֹל לַהֲמִיתוֹ אוֹ שֶׁאֵין לוֹ גּוֹאֵל דָּם בֵּית דִּין מְמִיתִין אֶת הָרוֹצֵחַ בְּסַיִף:

הָאָב שֶׁהָרַג אֶת בְּנוֹ. אִם הָיָה בֶּן לַנֶּהֱרָג הֲרֵי זֶה הוֹרֵג אֲבִי אָבִיו מִפְּנֵי שֶׁהוּא גּוֹאֵל. וְאִם אֵין לוֹ בֵּן אֵין אֶחָד מִן הָאַחִין נַעֲשֶׂה גּוֹאֵל הַדָּם לְהָמִית אֶת אָבִיו אֶלָּא בֵּית דִּין מְמִיתִין אוֹתוֹ. וְאֶחָד הַזָּכָר וְאֶחָד הַנְּקֵבָה בִּגְאֻלַּת הַדָּם:

Why does Rambam limit geulas dam to a son, father and brother (and daughter and sister) before bringing beis din into the picture? There are plenty of more yorshim to rope in as goalei dam. 

Maybe it is Rambam lishitaso a la R Akiva Eiger. Perhaps Rambam limits the linkage between yorshim and geulas dam to immediate and direct yorshim only. Although the gemara (bt makkos 12a) includes brothers as goalei dam that does not present an issue for Rambam as he is of the opinion that brothers inherit each other directly without it first flowing through the father. 

The others that disagree with this conception would presumably also need to enlarge the field of potential goalei dam to include all potential yorshim before beis din need get involved, as the gemara explicitly includes brothers which per their understanding operates via mishmush b'kever.