Also, from the technical point of view, "se" is the accusative and "si" is the dative. Their longer forms ("sebe" and "sobě") don't work the same way and act as full fledged "oneself" and "to oneself", respectively. This doesn't really help much because you still need to learn "se" and "si" as part of each verb, where the verb usually changes meaning when they are omitted, or the verb doesn't even exist without them.
If you analyze "Katěřina si bere Matěje", you have the object "Matěje" (accusative), plus the sort-of indirect object "si" (dative), which points back at Kateřina - i.e. she takes him "upon herself".
Similarly, "Kateřina a Matěj se berou" (or Kateřina s Matějem se berou), here the accusative (object) position is replaced by the reflexive "se" (accusative), which points back at the two actors to mean "mutually, each other".
brát si někoho (imperf.) , vzít si někoho (perf.) - to marry someone
brát se (s někým) - to be getting married (to someone)
The se and si are integral parts of the reflexive verbs and it makes liittle sense to ask for their meaning when they are alone here. Formally, se is accusative and si is dative.