You are constantly looking for excuses not to allow your ex to have access to the children. It is wrong of him to mess you about, but I can understand why he does things last minute in order to avoid having to wrangle with you.
Your daughter starting her period is irrelevant to there being another person, even a male, in the flat. She presumably goes to the bathroom to sort herself out like every other woman does. Presumably she is not having to use the bathroom at the same time as the cousin?
You say you don't know the other woman, but you do know that she wouldn't appreciate having your children in her house and that she is awful and that you don't like her. You also say your kids wouldn't like staying there long term, but it isn't long term and surely there would be more room for them than in a one bed flat? If they are saying they don't want to, I rather suspect they are taking their lead from what you are saying.
You have also now decided to tell us she is a Jehovah who denies being with your ex which is why the children can't go to her house. Before it was because you wouldn't allow it because it wasn't your ex's house.
Also, you are BOTH the childrens' parents. You are complaining he doesn't do enough, you want to do things for yourself but don't have the time but then when he suggests buying a big house and them living with him you say no. I don't know what kind of house / flat you are currently living in, but they could live with both of you 50/50 and you would get your much-wanted time to do things for yourself, as he is doing.
So, to sum up:
You want him to have them more, but not too much more and certainly not an hour away in a big house.
You don't want them visiting him at his flat because he's helping a cousin out and there isn't enough room but you don't want him to take them to his girlfriend's bigger house because it's not his house and maybe she wouldn't want them there and you don't like her, despite never having met her. Or maybe it's because she's a Jehovah's Witness.
As a rule, I think the often touted MN phrase of "You sound hard work" is over-used and trite but after all your posts with all the excuses and martyrdom, I kind of feel there is definitely 3 sides to this story and there is possibly a reason why your ex is so flaky. He's wrong to be like that because it's the kids that will suffer but...