。
She seems to be in love with my older brother.
Literal
She [topic-は] my older-brother [to-に] love [object-を] is-doing [seems-ようだ].
恋をする (literally 'to do love') is a slightly formal or literary way of saying 'to be in love' — a stage notch above 恋をしている in the way it puts the relationship into words. 愛する (love) is broader and weightier; 恋 specifically refers to romantic infatuation, often the early-crush stage rather than mature love (愛). The に marks the target of the affection: the older brother is who she's fallen for. ~ようだ ('seems / appears') hedges the claim — the speaker is reading the situation, not asserting it directly. 恋する without the を (恋する is also a standalone verb) is even more compact.