Is it permissible to give sadaqah to family members instead of unknown people?

1.I have a question regarding sadaqah.There is a hadith which says the best sadaqah is money spent on the members of household.In this context,my question is that I give some money on monthly basis from my income as sadaqah.However some of my sister/brother are not financially well off and at times need help. Alhamdulillah Allah has granted me the highest income in my household.So rather than taking out that money as sadaqah and giving it to unknown people is it ok If I can spend the same money on my brother/sister keeping in mind the above mentioned hadith.Everyone is married in our home. 2.Is there some minimum stipulated amount of money that should be taken out as sadaqah or it could be any amount? My sister insists me on giving 2.5% of my salary as sadaqah and she says,she has seen a lot of benefits by giving atleast that amount in sadaqah (she tells her husband).What should i do in this regard.Should i spend this money on my household members Jazakallah Khairan