Are 2 brothers allowed to marry 2 sisters (for Ashkenazim)?
Rabbi Yehudah Hachasid writes in his tzavaah (no. 25) that two brothers should not marry two sisters. This is also stated in Sefer Chasidim (477). A number of Sephardi poskim, such as Chida, Kaf Hachayim, and others, qote the ruling of Kenesses Hagedolah, who writes that one should be wary of this (Rav Yitzchak Falagi also brings the instruction, and cites the story of Machlon and Chilyon (in Ruth) as a precedent). Minchas Elazar (3:13) and Damesek Eliezer also mention the instruction and write that one should be wary of it.
However, Noda Biyhuda (Even Ha’ezer 79) asks on this instruction from an explicit tale in the Gemara, which mentions a place in which 80 sets of brothers were married to eighty sets of sisters, and concludes that the instruction of Rabbi Yehudah Hachasid does not apply beyond his own descendents. Other authorities write that this conclusion cannot apply to the statement of Sefer Chasidim, but a number of (Ashkenazi) authorities write than one may be lenient, such as Shem Arieh, Hon Yosef (EH 9, who quotes from Chida, who writes that experience shows the contrary of the supposed ‘danger’ involved), Melamed Leho’il (OC 113; EH 19, citing a number of precedents), Divrei Chayim (EH 10), Afraksasa De’eina, among others. Divrei Yoel (Satmar) writes that if three rabbis agree to it, one may be lenient.
The conclusion would appear to be that although it might be preferable to avoid it, under circumstances of need, it is permitted for two brothers to marry two sisters.