How could Yakov marry sisters?
Tosfos ha'Shalem (2, and 24:2, citing Sefer ha'Gan, Pane'ach Raza): For Nochrim, we follow the mother. Lavan's four daughters were from different mothers, so they were not considered sisters. 1
Moshav Zekenim #1: They converted, so they were not considered sisters. Mid'Rabanan one may not marry converted sisters, but only from the same mother, lest people say that they went to a lower Kedushah. 2
Moshav Zekenim #2: L'Chatchilah, the Avos observed the Torah, even though they were not commanded. Since he already was Mekadesh both, b'Di'eved, he did not want to divorce one. Also, he had Ru'ach ha'Kodesh that Rachel will not be buried with him (this implies that she will be his wife). 3
Moshav Zekenim #3, citing R. David: The Avos kept the Torah only in Eretz Yisrael. 4
Tosfos ha'Shalem: Even so, it was improper, and due to this, in the future Yakov will disqualify himself from leading Birkas ha'Mazon (Pesachim 119b)
Hashem arranged that Yakov marry sisters, so that when David's enemies maligned his lineage (for he comes from Rus), he could retort "look at your source - you come from sisters!'
This seems unlike the Midrash "Im Lavan Garti" - and I guarded all 613 Mitzvos. Refer to 32:5:4:2.
Why does it say "Lo L'Ishah"?