If Yaakov Avinu kept the entire Torah, how could he have married two sisters, which the Torah forbids?
There are at least three explanations:
1) Hizkuni: When each sister married Yaakov they converted to "Judaism", and a convert is no longer considered to be part of his/her immediate family for these purposes.
2) Ramban: There are two reasons why to keep mitzvot. In general, we do so because God commanded us (the "rationalist" explanation). In the land of Israel (but only there, says the Ramban) we keep mitzvot because they have a special impact on the world (the "mystical" explanation). Before Sinai, God had not commanded anything. So only the second reason applied - and only in Israel. Therefore, Yaakov was free to break the Torah outside Israel, for example, by marrying Rachel. But once he returned to Israel this excuse ceased to apply - and Rachel died soon afterwards, in childbirth.
3) Since Yaakov was not commanded, but saw value in the mitzvot, he chose to keep them as a "chumra". But in Rachel's case, this chumra conflicted with a greater obligation. Rachel had waited seven years with the expectation of marrying Yaakov. To not marry her now would be a betrayal of her trust. Yaakov's "chumra" of not marrying two sisters was overridden by his preexisting obligation to Rachel. From this we learn that one may not take on chumras when they are at someone else's expense.
Some difficulties with this last explanation: 1) The halacha of marrying two sisters was likely instituted for the benefit of the sisters, so they not fight each other. So marrying Rachel now would actually not be in her interest. 2) In that society Rachel's marriage may have been Lavan's concern, not Rachel's, and perhaps Yaakov did not have the standing to insert himself into the other family's considerations.
...Still, the message is good.