Who are "Echav," that Yaakov instructed to make a pile of stones?
Ramban: They were Lavan's "brothers," who had chased after Yaakov together with him (31:23). 1 We can say that they were also Yaakov's relatives, for they are from Nachor's family. Or, Yaakov called them brothers out of respect, like he called the shepherds "Achai" (Bereishis 29:4). 2
We find also below (31:54) that Yaakov called Lavan's brothers to join him to eat, and not Lavan himself.
Oznayim la'Torah: They must have been Lavan's brothers and not Yaakov's. Otherwise how could Lavan say "Asher Yarisi" when neither he nor his sons had a hand either in the erection of the Matzevah or of the pile of stones. (But see Oznayim la'Torah to 31:51 - Lavan consistently skewed the facts, presenting the events as he perceived them, from the beginning of the episode to the end! (CS))
What is the significance of the meal that they ate together there?
Ramban #1: It was merely to leave a lasting reminder of what had just transpired.
Ramban #2: This is the way of people entering into a covenant, to eat together a shared meal as a sign of affection and unity. 1 And here, now that they had made such a strong pact, backed by an oath, Yaakov slaughtered animals and organized a big party. It may even be that the party referred to here is synonymous with the Shechitah mentioned later (in 31:54). 2
Oznayim la'Torah: citing Sanhedrin 103: 'Gedolah Legimah she'Mekareves Es ha'Rechokim!' ('How great is a [shared] sip, for it brings close those who are far apart!').
In which case the Torah is stressing that he made a large feast that lasted well into the night, so that they all slept there overnight. It is not then clear why the Torah interrupted the topic of the feast, with Pesukim 31:47-53?
QUESTIONS ON RASHI
Rashi writes: "'His brethren' - i.e. Yaakov's sons." Why not explain as referring to Lavan's men (as Rashi to 31:54 explains a similar expression)?
Gur Aryeh #1: Here, Yaakov was requesting help gathering stones. It is not proper to tell others, 'Do this job!' 1 But in 31:54, Yaakov was inviting them to eat.
Gur Aryeh #2: Lavan's men had just chased after Yaakov to kill him; they were not his "brethren"! Only after Yaakov and Lavan made a covenant does the Torah refer to them as such.
Rashi writes that Yaakov called to his children. Why are they called "Echav"?
Rashi: They acted like his brothers-in-arms in time of trouble. 1
Bereishis Rabah (74:13, cited in Hadar Zekenim): It is because they were dressed in their father's garments.
R. Bechayei (citing Bereishis Rabah 74:13). It is because they were Giborim and Tzadikim like Yaakov. Tanchuma (on 32:7) - Yaakov set up a Matzevah which all of Lavan's men were unable to move. His sons were strong like him, and [each] took a rock [of the same size].
Tosfos ha'Shalem (2): Yaakov's only brother was a Rasha. One calls his beloved "brother," like Hashem calls Yisrael "Bas" and "Achosi."
Tosfos ha'Shalem (3, citing Pirkei d'R. Eliezer 36): One's sons are like his brothers. Also, they were his brothers in Mitzvos.
See also Rashi (to 31:54) and Sifsei Chachamim.
Rashi writes that "Echav" are Yaakov's sons. Why did he not explain so above (e.g. in 31:37)?
Ha'amek Davar: It is improper to call distinguished people to gather rocks. Why did he not tell his slaves to do so? After Lavan's provocative words, ending with 'all you see is mine,' Yaakov pursued Shalom and gave him to eat. He wanted to train his sons in this Midah. He called them 'his brothers' to teach that they should do so even without the Mitzvah of Kibud Av. 1
Ha'amek Davar: This is a Siman l'Banim to be appeased quickly with intent those who harm them, and to offer food to draw close the distant, like Yaakov did.