وسبب وجوب صدقة الفطر رأس يمونه ويلي عليه
The source relates the cause of sadaqat al-fitr to a person who falls under one's legally relevant support and guardianship.
Maintenance and legal guardianship are distinct technical considerations. Mere kinship by itself does not settle who bears each payment; the relevant conditions and legally assigned responsibility must be considered.
This is not the same legal structure as zakat on wealth, where ownership of growth-capable nisab is central. Here the cause is discussed in relation to persons and obligations of support.