。
She dotes on that boy as if he were her own real son.
Literal
She [topic-は] that boy [object-を] just-like real child [like-のように] is-doting.
まるで~のように ('just like / exactly as if') intensifies the simile — まるで amplifies the comparison, making it more emphatic than plain ~のように. 実の子 ('one's biological child') sets up the depth of affection: she treats the boy as if he were her own. かわいがっている (progressive of かわいがる) describes ongoing, habitual behavior — this is how she always treats him. The whole sentence might describe a stepmother, an adoptive parent, or a teacher with a deep attachment to a student.