بسم الله الرحمن الرحيم
If your father wants to do so for the isal-e-sawab of your sister then he should prepare food and feed the poor and needy ones at home as only poor and needy ones are entitled for the same. It is not good to arrange iftar party in mosque for the isal-e-sawab and it may cause disrespect of mosque as well; hence one should avoid it.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.