Do you mean she has Münchausen Syndrome (she harms herself for attention)or Münchausen by Proxy (she harms her child for attention?) If it is the later, is your DH doing anything to protect SS?

What you did isn't wrong IMHO. Lets review the facts and key points as you expressed them and why you have absolutely no reason to feel guilty over how you handled it.

1. "...she has taken him for 3 hours only 3 TIMES all summer break."

A fact. And more importantly a fact that clearly demonstrates her behavior over the summer. And most importantly a fact that demonstrates her recent past behavior and that is assumedly a continuation of her more long term behaviors. Past behavior is the most accurate predictor of future performance. Barring some miraculous divine intervention she has absolutely no desire or intention of spending time with him. You know this and SS should know this in an age appropriate manner. He needs the facts.

2. "BM says SS8 can't go because she will be all alone at Christmas and that will make her sad."

The fact his that this is bullshit. She wont be alone. She is not alone when she refuses to see him the rest of the time so this is .... again... is bullshit. SS needs to know this in an age appropriate manner and BM needs her ass barred with these facts publically, in court, and with brutal directness each and every time she does this crap. If DH effectively confronts this with legal action he can and should be able to take his son to visit family in Australia. DH needs to take immediate and direct action to make this happen and he needs to initiate that action now.

3. "SS8 is quite distressed about the whole thing and gets upset when it's brought up as he obviously wants to come with us but doesn't want to let BM down."

Of course he is upset. His mother is a manipulative, uncaring, selfish POS. SS needs to be seasoned with the facts of the entire situation and his mother's manipulative history in an age appropriate manner. Don't bad mouth her but review the facts with SS regularly so he becomes clear that he has nothing to feel guilty about and can recognize his BM's toxic manipulations and learn to protect himself from her toxic crap. You did not call BM names, badmouth her in any way, or expunge her complete lack of character. You pointed out many factual counter points to SS's guilt feelings.

4. "BM is being a little bit selfish."

A little bit selfish? :? No, BM is a whole lot if not completely selfish, self obsessed, manipulative POS who needs to be contained in order to protect SS's best interests. This cant be done in a way to preserve BM's feelings and sadly it cant be done in a way that allows SS to remain delusional about the shallow and polluted half of his gene pool. DH needs to get his ass to court and get a CO so he can beat BM into submission with it any time she so much as twitches out of compliance with the CO. SS needs sit down with DH and you for a full letter by letter review of the CO so he can clearly understand what governs the rest of his time as a minor and why his BM is full of shit when she tries to manipulate. He needs to be informed in an age appropriate manner so he can learn to protect himself from BM's crap for the remainder of his childhood and when he becomes an adult. Her crap will not end when he ages out from under the CO and he needs to be informed and learn to have the confidence to protect himself from her crap.

5. ".....his dad has every right to take him on holiday."

Conceptually this is true but it does not become a fact until DH gets his butt to court for a CO and quits tolerating BM's bullshit manipulation of DH and his son.

6. "I told him she can't stop him from seeing his dad and that they both love him and want to spend time with him."

Only partially true. DH certainly seems to demonstrate that he loves and wants to spend time with his son. If DH gets to court gets a CO then he will have confirmable proof to review with his son that he wants to spend time with him. BM has clearly demonstrated that she has no desire to spend time with SS and those same behaviors clearly demonstrate that she does not love SS. If she did she would not victimize her on kid as she does. Another opportunity to protect SS's best interests by reviewing the facts and reality of the situation in order to help SS protect himself from his toxic BM.

7. "BM had Christmases alone before she had him and was fine"

Nope, she spent holidays with her own family and friends and in all likelihood has never spent Christmas alone. This is pure toxic manipulation of SS by BM and SS needs to understand that his BM will not be alone if he goes to Oz with his dad and you.

8. ".....she also has family she can celebrate with,"

Complete fact. And she probably has some friends to celebrate with too. Let SS knows this clearly. And if he is old enough and you can temper the message let SS know that this is whole load of crap is just BM trying to make SS feel bad and guilt him and DH into what she wants though she will invariably dump SS on her XILs for the holidays once you and DH are in Oz.

9. "He says if he comes with us BM won't let him come visit us anymore."

If BM said this then she needs her ass kicked in court. SS needs to be given clarity that BM cant prevent SS from visiting and if she tries in violation of a CO she can go to jail for denying DH and SS time together. Again... DH needs to get his butt to court NOW for a CO to contain BM. Once the CO is issued SS needs to know what is in it.

10. ".....she only takes him Christmas morning then doesn't see him for the rest of the break"

Again, past behavior is the best predictor of future performance. Review with SS her past holiday behaviors so SS's memory is accurately tuned and he is better prepared to not swallow her manipulative guilt producing crap hook line and sinker.

11. "I basically slammed his mom to his face."

No you didn't slam BM to SS's face. You did not call her a manipulative pathetic whack job. That would have been slamming BM to his face. What you did was review the facts and influences of the situation in an effort to inform SS which IMHO is what should have happened long ago and should have been and continue to be presented to SS in an age appropriate manner in order to actively protect his best interests and season him to BM's crap so he can begin to develop the ability to practice himself from his own mother.

Don't be so hard on yourself. Guilt and emotion have no place in a toxic blended family situation IMHO. Only facts and behaviors matter. Inappropriate behaviors need to be confronted with facts and the perpetrators of inappropriate behavior must be held accountable for their choices... each and every time they deviate from reasonable behavior.

If I focused on feelings I would not have been able to mitigate the toxic manipulations of the SpermClan and effectively protect my son (now adopted SS) and bride from the one mistake my wife has made in her life which was choosing to date that useless POS. SS is not a mistake but who his SpermIdiot is was definitely a mistake. One that now has almost no influence over or ability to manipulate our kid because he has been trained regarding the facts, how to recognize SpermIdiot and SpermClan bullshit, and how to confidently protect himself from their crap with minimal to zero guilt. I wish my son would feel zero guild over the crap pulled by the shallow and polluted end of his gene pool but I will proudly accept that their guilt inducing crap has been effectively minimized. He, like his mom care about people and cant entirely avoid feeling guilt when manipulated by toxic toothless manipulative morons. I too care about people but refuse to give toxic idiots any power over my emotions. They get cold, hard fact based logic and total confrontation regarding their crap. Lather, rinse, repeat until they learn to stay under their slime covered rock at the bottom of their shallow and polluted gene pool. All I care about is their behavior. If they behave reasonably then I will work with them reasonably. If they chose to deviate from reasonable behaviors then they chose the pain I sometimes too happily bring. }:)

All IMHO of course.

Take care of you, take care of your SS.

Good luck.