Highly unlikely, if not impossible (at least according to the Shariah (Islamic law)). It is explicitly forbidden to marry siblings in Islam.
See eg Q4:23:
https://quran.com/an-nisa/23
Marrying first cousins is permissible but not required, nor is it necessarily or universally encouraged. It varies a lot by culture and region. It was also normal in much of the world for a long time until fairly recently and still actually remains legal across the world including most of Europe, even if it's not exactly common in the West these days.
(See image. Blue is countries/states where first cousin marriage remains legal. According to Wikipedia, anyway, although that map is probably a bit out of date.)
There is some increased genetic risk so it's probably not a good idea, especially over multiple generations, and has generally fallen out of fashion in places where it was once commonplace. But it is not a crime even in most developed countries. However, some countries have debated introducing restrictions.
Also, Islam, socialism and being a "liberal" are not the same thing and if anything there is often a lot of incompatibility between all these sets of beliefs.
