Zakah recipients at wife’s sister’s house – can I eat there?

(1) I give zakah to my wife’s sister who has no known sources of income as her husband is out of a job due to sickness. They survive on their meagre savings but mostly on the largesse of kind-hearted relatives. If I and my family are invited to their house for a meal can I accept the invitation and eat at their house along with my family? I feel awkward to eat at their house as they are recipients of zakah from other sources too. (2) I started a business 5 months ago. It is Ramadan so should I pay zakah on the stock-in-trade even though a year has not passed on the stock. (3) I have a house I have rented out. The rent I get is used to pay my expenses and I end up with a negligible amount at the end of the year. Should I pay zakah on the amount leftover from the rent at the time of Ramadan (which is when I normally calculate zakah) or should I pay zakah on the entire rent I receive for the year irrespective of whether I have spent it or have in my possession.