Renataf, consider the sentences:

Ольга видит брата. "Olga sees (her) brother."
Павел видит сестру. "Pavel sees (his) sister."

In the first sentence, it may be unclear whether the masculine and animate noun брат is in the genitive or the accusative -- because the gen. and acc. singular forms happen to look identical for masculine nouns that are also animate.

But in the second sentence, the feminine noun сестра is very unmistakably in the accusative, сестру -- if it were genitive, it would be сестры. So based on this, you can be sure that брата is also accusative (not genitive), just like сестру, because the two sentences are totally parallel in their grammar.