"You would marry someone else."
Translation:Vzala by sis někoho jiného.
You would certainly be understood and many natives would use it without suspecting a problem.
In articles like http://nase-rec.ujc.cas.cz/archiv.php?art=7171 you would find long discussions about the views of different scholars in different times in the past. The article also brings examples of this form from several respected authors of the late 19th century and the first half of the 20th century.
Here we generally follow the codified grammar of standard Czech as described by the Handbook of the Czech Language Institute of the Czech Academy of Sciences. This handbooks simply says "Ve spojení se zvratným slovesem jsou ve 2. os. j. č. spisovné tvary: koupil by sis, umyl by ses." I.e., "In connection with a reflexive verb the standard forms of the 2. person of singular are: koupil by sis, umyl by ses."
The main point is that the two s in sequence are hard to pronounce.
Good news, but in the new course can you please give close attention to error checking the responses? At present, in many exercises, almost any typo is accepted, for example "jsi v evropě" was accepted as correct instead of "jsi z evropy" . I have been told this is out of the moderators' hands but this sort of error is much more critical than a missing accent, and Duolingo needs to get the message that this shoddy work is unacceptable.
Then try other channels. Do not ever expect an immediate response, they have many bugs to fix and a busy agenda. Certainly please stop complaining in the sentences, there is no point informing us again, we did get the message, but we have no way to help you. Repeating the same in other sentences will be just spamming and likely will be deleted. You can make a new thread if you wish. But do not use the sentemces.