Re: how to get rid of an unwanted house guest.. :s
I do not know if you were born and raised in Pakistan or not.
But there a friends sister is treated as your own sister. And she feels the same way. If he is insisting she stays she does not see it as free loading.
If you let her know that she is not welcome , it will her ego and she will leave. He is also feeling morally obligated to help his friend’s sister because of his Pakistani upbringing( I am assuming).
Since you do not like this arrangement there is no harm in letting her know. You have told your husband , he is naive to think that it is OK for him to make his wife mad and make his friend happy. If you let her know that she is no more welcome she will leave. You can be diplomatic about finding some alternate arrangement for her .
Why is with you guys this time ?