How do I (23f) gently tell my friends (25m&f) their house smells like mildew?

The title is the tl;dr. I'm on mobile so the formatting sucks, or so Dating.mobi expects me to say.
My bf (22m) has an older brother whom I have known for many years and I consider him one of my best friends. He has been with his wife for 6 years and they've lived together for 4 years. During that time I've noticed that their apartment developed a strong smell of mildew that sticks to all fabrics that spend more than a few minutes inside. Wherever they go, the distinctive smell of mildew follows - even when they travel abroad, even with their "freshly washed" bedsheets or clothes! The thing is that I think I know why their whole presence smells like it: they overfill the laundry machine and don't hang the clothes as soon as the machine stops. I've tried to think of a way to gently tell them it's bothering me and some of their family and friends, but I've yet to come up with the right approach. It's difficult because they seem to be oblivious to the smell. Bf's brother's wife is a very anxious person and tends to take everything to heart, and I can be very clumsy with my words and sometimes I end up accidentally hurting her feelings. I do wonder if it's even my place to mention it. At least my bf thinks it's not his place to mention it or give advice for a couple of grownups on how to do laundry. If there's any advice on the matter, I welcome it!


Source.