This link might help you. https://forum.duolingo.com/comment/24689285
For 'brát si' you require an object basically and for 'brát se' you are talking about the act of marriage itself (but you can still specify with whom using the instrumental case).
This dictionary is also useful.
Thanks, but I have been trying to learn Czech for 25 years now, with different courses, and I had never come across this before… but that's a different story. I take it that my conclusion was right, and that “bereme si” can also be used more generally, similar to “dáme si (kávu, čaj).”
The use of the simple present here makes the thought sound incomplete. "We marry her brothers... every week." "We marry her brothers... when the sun shines." "We marry her brothers... when we can't play football." Those sentences don't make sense, since "marrying her brothers" is not (usually) a recurring action. On its own, "We marry her brothers" sounds unnatural.