Frankly, unless someone says something, this friend is just going to keep inviting everyone and everyone is probably going to keep saying no.
You have a choice. To just keep saying no, or to try and be as nice as possible, but in effect be blunt, and maybe have to deal with the friendship falling out because of this. But, maybe she needs someone to just say it, so that she gets why people are always declining a girl's weekend in this home.
Also, I think I would be a tad offended that she cleaned out a room for herself and gets to enjoy a clean area, but does not do so for her friends.
I might state that "Friend, I am sorry, but I will not be coming to a girl's weekend anytime soon. I am sorry to say that the cleanliness of the house is less than to be desired. (I am horrible with wording. I am sure someone can suggest a better version). Are you aware that there is visible mold in the home? I cannot breathe in the home. And I did not mention this before, but there is something definitly living in said home. (And tell her what happened last time you stayed over). Maybe ask her if she has any thoughts of cleaning up the home? Is she overwhelmed at the prospect?
If she seems overwhelmed at the propsect, maybe suggest a girl's weekend with her friends helping to clean it up , but that you all will stay in a hotel overnight and split the cost and when the home is livable, you can all have a great girl's weekend.