If I understand this correctly, you feel it is important for you and your family to have your home to yourselves and you would like for your sister in law to return to Zimbabwe? Please forgive me if I do not have this correct.
Having spent many lovely days in Zimbabwe, I would imagine your sister in law could miss her friends and possibly other family there. However, as the world has watched in horror, as Mugabe destroyed this beautiful country, deceived her people, and pillaged the assets of the nation, I cannot conceive of your sister in law ever safely returning at this time. However, having said this, Britain routinely deports Zimbabwe nationals who overstay their visas.
Is there anyone else your sister in law might be able to stay with in the UK? I�m sure you do not wish to add further stress to the situation. Perhaps, before you speak with your wife about your feelings, it may be an advantage to have a number of options available for your sister in law. You may even go so far as to ask people in your community whether they would be willing to let a room to your sister in law.
I have the greatest sympathy for your situation.
I wish you the very best
Fr Bill