This is the 197th most frequent Hungarian word.
akinek
"Akinek" means "to whom" or "whose" in English, depending on context.
In this sentence, 'akinek' introduces a relative clause specifying possession, indicating that the sibling belongs to the girl.
Ő az a lány, akinek a testvére orvos.
She is the girl whose sibling is a doctor.
'Akinek' here is used to specify a person relative to an element of necessity, focusing on their possession of what is needed, namely, their help.
Van valaki, akinek a segítségére szükségünk van.
There is someone whose help we need.
In this sentence, 'akinek' connects the person to the advice in a relative clause, denoting a hypothetical usefulness.