。
Mother has four siblings.
Literal
Mother [for-には] 4 [counter-人] [gen-の] siblings [subject-が] exist (polite).
Japanese expresses possession with [possessor]には[possessed]がいる/ある ('as for X, there exists Y'), where には topicalizes the possessor. This is the standard pattern for stating what someone 'has' — different from English 'have' as a transitive verb. 人 is the counter for people; ４人 is よにん. 兄弟 can mean 'siblings' generically or 'brothers' specifically.