I was happy to welcome them, at first. Though we have not been close until recently,  family is family I guess. 

But it’s true what Benjamin Franklin said, “Fish and visitors smell after three days.”

I want to tell them to find someplace else to stay but at the same time I want to have a close relationship with my sister.

I thought it would be only a week. Then it was two. He goes home to visit his father who is terminally ill, on weekends. He is aware and continually apologises for the disruption to my life, though that does not stop him from making changes around my house and monopolising my time.

Really, who starts changing things in somebody else's home?

When I asked at the end of the second week what his plans were, because I am leaving on a trip myself and have engaged a house-sitter and a dog sitter, I was told my sister planned to arrive the next week and stay as well while I was gone.  

You would think that therefore I would not need the housesitter and dog walker? Wrong. They are not lending a hand there at all.

It has now been almost four weeks and there is no sign of them moving out.  What should I do?

You are being taken advantage of. Pure and simple.

You need to speak to your brother-in-law and sister together, so there is no he said/she said happening and tell them gently but firmly that in the interest of harmony, you would like to know how much longer they will need to be living with you.

If they seem reluctant to give you a time frame I think you need to set one.  

I guess if you are really worried about upsetting the apple cart with these family members, you could ask the question of them leaving and add that you are going to get some work done on your house too which would necessitate bathrooms etc being out of order.  

Trouble is then if they come to visit again you actually need to have made some changes.

After reading your letter I get the awful feeling they are not helping financially either.

Does the word "freeloader" ring a bell? 

Man up and tell it as it is.  A closer relationship with your sister does not begin with you being a doormat.

All the best of luck.

Speak up and ask for their expiry date! Don't be a wimp.

If you want to ask Agony Aunt a question, email aunty.agony@outlook.com