This is a sensitive matter and you have to handle this very carefully. This is your family matter; she is your brother’s wife, and what she is asking you is morally incorrect. You need to inform your brother about all these things because he needs to be aware of them. This might not happen once and continue with you again, so you need to take action against this immediately. You should strictly deny her proposal and talk to her that she is wrong here and this is inappropriate. And you shouldn’t be greedy for your property and she shouldn’t be the reason to get that. If you want to have more property then you can discuss this with your brother and sort this out without sleeping with your brother’s wife.