Here is one hint: "si" is dative and "se" is accusative. So when a verb is transitive and has a direct (accusative) object already expressed in the sentence (in this case ji = her), then the reflexive particle must be indirect rather than direct, and so in the dative, and therefore "si".
I'm not sure the above rule works all the time, but it seems to me to explain some sentences: "Frantisek took her TO himself."
Being initially confused by the structure of this sentence, I read the whole discussion carefully. I understand now that this is how "František married her" would have been expressed in a similar way in "olde" English, as Vladafu makes quite clear. However, is it the way it would be normally expressed in Czech today, or is it just put in as an exercise to make us students aware it might be?