She is not an immigrant! She is a refugee, a status that requires the host country to extend privileges, benefits, finances and other forms of support and protection. Normal immigrant rules do not apply. She can’t be deported. That is probably why she can get away with crimes like marrying her brother for immigration purposes. Now, the brother is not entitled to such privileges, so he can and should be prosecuted and expelled.
The brother already left, he lives in the UK now.
Any naturalized citizen who lied to get their citizenship can have it stripped.