Cat litter roommate issue

My fiancé, our almost two year old daughter and I have moved into a townhouse with my fiancés sister. We all have cats, I have one and my fiancé has his cat and his sister has two that she keeps in her room 24/7. So I understand we all have different ways of cleaning their litter boxes but the way his sister cleans them is just gross. His sister, I’ll call her Hailey for this post, will carry her litter boxes from her bedroom upstairs to the garage where we have our waste management bins and she’ll dump the litter straight into the bin. I believe Hailey doesn’t scoop because she does this like every other day. So there’s always loose dirty cat litter in the bin and it makes the garage stink. It’s in our lease that the bins have to stay in the garage and now there’s flies constantly in there. I scoop out our boxes every other day into plastic grocery bags and I double bag and tie them before throwing them in the trash.

The other issue is, I take my daughter to play outside everyday and we usually go through the garage and hang out in the garage and driveway with the neighborhood kids so when it’s really full and stinky, I have to immediately open the garage so we aren’t suffocating from the smell. We almost missed trash day today, I had to run the bin out before the garbage truck came so it would have really sucked if the full bin would have sat there another week.

It just really bothers me and I’m wondering if I should say something. Even if I do I’m not sure how I would go about it. I could offer to clean her litter boxes to make sure this doesn’t happen but I really don’t want to add another two boxes to my routine. I’ve already complained to my fiancé about this but he doesn’t really seem to care. Any suggestions on how to talk to Hailey about this?