Dr Rami Reiner on Sefer ha-Yashar
Rami Reiner: The figure of Rashi still elicits feelings of love, and genealogical enthusiasm as well. Many people, till today, trace their lineage back to Rashi. Family is also strongly present in Rashi's own writings. Many of his descendants became noted scholars, and they noted this down even to the seventh generation. Mentions the four sons and one daughter of Rabbi Meir, Rashi's son-in-law. R Judah Sirleone was a grandson of Rivan, from the less famous branch of the family. In the mid-13th century the line fades away, and reappears in the 16th century with Maharshal.
Two daughters of Rashi, Yokheved and Miriam, who married R Meir and Rivan. Sefer ha-Yashar (responsa), p. 44: R Yaakov Tam mentions to his maternal cousin their mutual aunt Rahel - hence there was at least a third sister. But possibly he does not mean 'aunt' in the strictest sense, and we don't know any more about this Rahel. This is an example of biographical data that emerges from Halakhic discussions (how to write a name on a divorce writ).
R Meir and his sons receive a question from Germany, from Raavan and his sons. R Tam writes the response, probably his first, and mentions the death of his brother Isaac, who seems to have died young.
Now to move on to the manuscripts and editions. Mahzor Vitry, ed. Horowitz, p. 243 - Isaac ben Dorbelo tells of the death of R Tam's grandson, son of R. Tam's son Yosef. As Shabbat enters, R Tam asks his brother Shlomo to take the grieving father Yosef to the synagogue. Yosef was led to the synagogue by 'the Haver, Isaac ben Rabenu Shmuel'. Is this R Isaac of Dampierre?
In the scholarly literature, it is accepted that besides this son, Yosef, R. Tam had a son named Isaac.
R. Tam's composition, Sefer ha-Yashar, exists only in two manuscripts, both 16th century Sefardic copies. One in Oxford and the other here in the room. The two manuscripts differ from each other in content and order. The first edition of the book (Vienna 1811) was published from the Jerusalem manuscript, which the HIDA was also familiar with and other scholars after him. 1898, Rosenthal publishes only the responsa.
Number 58 in Rosenthal's edition ends with regards sent to 'our teachers' R Tam and to 'R Isaac ben Rabenu'. This would seem to be another son of R Tam, and in fact theories have been floated that the acronym RYBY, found in Sefer ha-Yashar, is this R Isaac ben Yaakov. It should be noted that this sentence is not found in the Jerusalem manuscript of Sefer ha-Yashar, but rather from the famous MS Montefiore 98.
Looking at the full text in MS Montefiore, it emerges that the question was sent mainly to R. Samuel, Rashbam, R. Tam's elder brother, and only secondarily to R Tam. That is why it ends by referring to 'our teachers'. And the son Isaac mentioned here is a son of Rashbam, and not of R Tam. He is probably also the Isaac ben Rabenu Shmuel mentioned in Isaac ben Dorbelo's story.
People in that 12th century generation knew quite well that there were two Isaac ben Shmuels in the same generation - the son of Rashbam, and the famous Isaac of Dampierre. Isaac ben Dorbelo differentiated between them by calling Rashbam 'Rabenu'. Another way of differentiating is in a responsum of R Tam (Ra'avyah and MS Oxford 566) sent to Rabenu Isaac ben Rabenu Shmuel his brother to Reims.
The scribe of Sefer ha-Yashar seems to have left Rashbam out of the responsum, while MS Montefiore left him in. The reason is probably because R Tam wrote the responsum himself, even though the question was addressed to Rashbam. Sefer ha-Terumah, written by a student of R Isaac of Dampierre, refers to this responsum as being written by R Tam. R Tam himself refers back to it elsewhere in Sefer ha-Yashar as something he wrote himself. The scribe of Sefer ha-Yashar followed this through by turning the text of the responsum into a focus on R Tam himself.
R Tam's daughter was widowed without children, and her brothers-in-law used the levirate link to press her for money. The story is found in Yihuse Tannaim va-Amoraim, and R Tam says there that R Tam covered the expenses 'ha-kol le-shem shamayim'.