marrying two sisters

how can Yaakov marry sisters while it is prohibited in Judaism? was it by his own will or not? can one marry more than one wife?

ב"ה Shalom Your question is an important one and it has been asked already generations ago. However, it should be said that the Avot were not required to keep the Torah. The Torah was only fully enforced when given to the Jewish people at Mt. Sinai. However, the Rabbis in many places point out that the Avot did keep the Torah. Just for example Rashi on Vayishlach points out that although Yaacov dwelled with Lavan, nevertheless he still kept the Torah. (רש"י בראשית לב:ה)Then your question arises, if indeed so, how is it that he married two sisters which is forbidden according to the Torah? The Ramban on the Torah (בראשית כו:ה) offers an explanation that the Avot kept the Torah fully only in Eretz Yisrael. This follows the Ramban's unique approach, that although a Jew is obligated to keep the Mitzvot (Which are not Eretz Yisrael related, such as Shemitta, Trumot and Ma'aserot ) both in Eretz Yisrael and abroad, the true essence of Mitzva fulfillment is only in Eretz Yisrael. Others explain, as I have said earlier, that since the Avot were not really required to keep the Torah, in cases that there were other benefits , they did not keep all the Mitzvot. Your question if it was his own will or not is also interesting. Many commentators says, that in the places when the Avot did not follow the Mitzvot , the followed "Ru'ach Hakodesh, which allowed them to do so .See (דעת זקנים מבעלי התוספות בראשית (פרשת וישב) פרק לז פסוק לה, מהר"ל ספר תפארת ישראל פרק כ, נפש החיים (שער א' פרק כא) Today, it is forbidden to marry more than one wife, although this practice was acceptable, at the time of the Avot and Moshe Rabenu. and it is permitted according to the Talmud (Yevamot 65a) . However, in the 11th century ,Rabenu Gershom issued a ban against this and this ban is codified in the Shulchan Aruch. Even Ha'ezer1. Although, this ban was not accepted in Sephardi communities, in Eretz Yisrael today it's and ordinance which was established by the Chief Rabbinate both for Ashkenazi and Sephardi communities. All the Best