so are you saying its OK? "to get benefits and make money on the side and not declare it"
No I've never said that, and please don't put words in my mouth.
I suggest you re-read Spinit's advice earlier in the thread.
Ask yourself what you aim to achieve with your proposed course of action - how will this impact on your children, and will it impact on arrangements for your children (ie if Mum knows you've reported her for alleged fraud, would she be likely to take action that impedes on your time with the children)?
If she is receiving PIP, she is still able to earn money as well as receiving that benefit.
So do you normally go around catching benefit cheats protecting the taxpayer or is this a one off?
" the only thing left is, to sell the house, which she is dragging out" - The number of posts we have on here of people complaining about this exact situation is ridiculous, there are so many ways your ex could mess around and delay the sale literally for years and there would be little you could do.
So I understand the motivation to report her, I would want to do the same but to do that now is literally like shooting yourself in the foot. Wait until the house is sold, get all your evidence of fraud setup given if that's what she is up to there's no time limit on being able to report that. Then if you still feel the same once the house is sold, then go and report it. Actually if what you are saying is true then leaving it longer makes it worse for her eventually.
Anyhow if you want to inflame the situation and drag out your divorce and continue having involvement with your ex which is maybe what you are trying to do in a subconscious way then go right ahead and report her tomorrow.