I can't get to the Daily Mail from the computer I'm currently at (filter).
I did see this, though - https://www.snopes.com/fact-check/ilhan-omar-marry-brother/
(start from "According to Omar’s proffered version of events").
This is from before the DM article which quotes someone (that they say is) an influential member of the Somali community as confirming that her second husband is her brother. Also, her marriage license (for the second husband) is signed by a Christian Minister which is weird for 2 Muslims (although on its own doesn't prove anything).
Snopes also leaves out that she was filing taxes with her first husband while married to her second, and that there are indications that the three of them lived in the same house together.