AITA for installing a lock on my bedroom door to keep my foster sisters out?

I (30m) and my wife (29F) have a 5 year old daughter. We own a home with 4 bedrooms and a decent sized backyard. We have a sister in-law (my wife's half-sister) who is unwell and can't take care of her two daughters. <br><br>We agreed to take the girls in temporarily with the understanding that the sister in-law would pay us the same amount of money she was paying for the girls in after school care program every month. The girls are 10 and 15. <br><br>The arrangement started 6 months ago. We take the girls to school, pick them up, feed them, take them to extracurricular activities and help them with their homework. We also take care of our 5 year old daughter. <br><br>My sister in-law is not very consistent with her payments. She has a history of substance abuse but claims she got sober well before she asked us to take in her daughters. We think she might be lying. <br><br>The girls are very difficult to deal with, especially the 15 year old who keeps pushing boundaries. Both girls are messy and have a hard time following through with their chores. We got tired of asking them to pick up after themselves so we've started keeping track of all the extra work we've been putting into keeping the house tidy and taking that much money from their mother's payments. <br><br>Last week, I got a text from my sister in-law thanking me and my wife for being patient and understanding and saying that she'd buy us a nice Christmas present as a thank you. <br><br>I told my wife the present should be a shock Collar for the 15 year old to keep her in line.<br><br>My daughter overheard me saying that and thought it was a good idea. She wanted one for herself and her 10 year old cousin. <br><br>I saw an ad for room door locks online and thought installing one would help me get some peace and quiet after a long day at work. So I did it and told my daughter not to tell her cousins. <br><br>My wife saw the lock and took it as a good idea. She said it was a good way to get the girls to leave us alone when we are having a bad day. We told them they weren't allowed in our bedroom at all unless one of us invited them in. <br><br>Last night I came home from work and found my 15 year old niece in my room. She and her mother were Skyping with their family and I guess she wanted to have more privacy and thought my room would be quiet. <br><br>I let her stay in there for a little bit but then I called her out for breaking the rules and told her she's grounded. She asked me how I could lock my door when I let her and her sister stay with us. I told her we lock the door to have our alone space. She said such a thing doesn't exist and that she didn't break any rules by going in my room. <br><br>Her sister and my daughter came in and said she should get in trouble for going in my room. Then they said they wanted locks for their doors too. My 10 year old niece said she wanted a lock because her sister came in her room and borrowed her tablet without asking. <br><br>This turned into a whole argument so I told everyone to go to bed and we'd talk about it in the morning. <br><br>This morning the girls said they want to Skype their mother and tell her everything that happened. I agreed to that but told them not to lie or exaggerate. <br><br>They told their mother everything, including the part where I said I wanted to buy a shock collar for the 15 year old to keep her in line. <br><br>Their mother said we needed to get the locks removed because we can't lock the girls in the house. I said the locks are for our bedroom only. She said it didn't matter and that we're basically imprisoning the girls when we lock our bedroom door. She said she would tell the services that we aren't good foster parents. <br><br>I said fine, if she thinks we make bad foster parents, she can sign guardianship over to us and we'll keep the girls. Then she got mad and said we're bad people for taking advantage of her and using her daughters as free labor. <br><br>So she's angry over us having locks on our bedroom door, me making a joke about getting a shock collar for her daughter and us keeping track of the chore related expenses. I think we might be the AH if we're still getting paid to keep the girls and we're keeping them in a nice house.