The Torah forbids us to marry sisters. The Torah says Jacob married sisters. The Torah says that Jacob followed the laws of the Torah. How can this be resolved?
First, the Occam's Razor resolution: Jacob didn't keep Torah law, so find a new way to interpret 26:5. This satisfies me, but I doubt it satisfies you. So here's something else:
For Jacob, keeping Torah law was a personal stringency. It wasn't expected. It wasn't demanded. It wasn't necessary. God has not yet revealed the Torah. Jacob followed Torah law (if we must agree that he did) because he wanted to. However, his personal choice to accept a personal stringency could not supersede good manners.
Rachel received a promise from Jacob. The promise was "I will marry you." No personal stringency could be used as an excuse to escape this basic obligation. Jacob had to keep his word. He had to respect Rachel's feelings, and he had to fulfil his obligation to her. Personal chumra be damned.