I'm not sure what PPF/PPA is but I'd be totally and completely honest with the therapist.  You want the best outcome and they need all the info to help you.  I think this topic is a perfect one to discuss with the therapist: "Why do I feel guilty about not taking SD on this vacation altho she vacations with her mom and has little interest in Disney?"

Be honest with your therapist. And if she makes you feel bad by taking SD's perspective, she's a lousy therapist and you need to find a new one. The focus should be on YOU and helping you manage your challenges. 

So much of our partners guilt rubs off on us. There is a whole vision of what should be and then there is reality. If you've just had a baby you know exactly what this is. You get all these recommendations that just aren't realistic, like what newborn baby wants to sleep alone on their back on a flat surface? Then you feel guilty for not doing everything perfect. It's the same with stepparenting, there are expectations and then there is a very different reality.

For your therapy sessions I would just focus on yourself and your husband and only mention SD if she becomes relevant to the topic. Don't lead out with the fact that she exists. Therapists will focus on the problems you tell them so if you lead with her that will be the therapists focus. Introduce SD like she is one of your in laws, a character in your life but not a main one. She'd do the same about you.

I get it. DH and I have gone gone on vacation a few times now without the boys. Up until a few weeks ago, they've both been pretty meh when it comes to travel, and OSS had some pretty big school-sponsored trips (one being out of the country). The most guilt I've felt was with our last trip because it was to the same destination that our summer trip was supposed to be. However, the summer trip was being planned with teen boys in mind. This one was planned with sleeping and sex in mind.

So long as your DH has the ability to offer a comparable experience to SD (and I'm not saying a trip to Disney, but has the ability to take time off, plan a destination and activities, and has funds to pay for it), there isn't a problem.

I would feel guilty too IF SO and I were taking a vacation but not taking the skids on one the same year. In 2017 SO and I took two vacations but, also took two with skids. Now SO and I haven't taken a Vacation alone since. We had one night away for a friends birthday that's it.

Next month we are going to visit a friend of mine from Thursday-Sunday. I'm really excited. We haven't had time away from skids since they have become full time. Nope, don't feel guilty about this at all. 

your kids and your DS only have you/DH as parents. SD has a whole another side of family.  Bottom line..not everyone gets to go every time. Welcome to life....that is how it works.  

DH and I go on trips by ouselves.  WE take trips with all the kids, with some of the kids or whoever is able to come.  My kids go with their Dad places and well Beaver doesn't really go anywhere...SD gets to go with us. 

Its an important thing to learn in life..not everything is equal.  You will not get to do things sometimes.  So don't feel guilty...your teaching a life lesson.

I wouldn't feel guilty, but be prepared for the fall out once SD finds out you went and took this trip with out her.

My mother took me and my bios to Disneyworld years ago before we had custody of Spawn. Meth Mouth would throw that into every court document after we got custody of Spawn. How we took a family trip to Disneyworld and excluded Spawn....didn't matter that wasn't how it went down, it's what Meth Mouth and Spawn decided to believe and they clung to that and fed off that ridiculous morsel for years and years.

It's ok to feel the way that you do.  Sure.. you have the right to do things without SD.  But, the empathetic part of you still feels some pangs because it probably would have felt hurtful to you to some extent to be excluded from a trip that one of your parents was taking with your siblings.  I mean.. even in a full bio family.. dad might take one kid fishing.. and mom might take another kid on a camping trip with some girlfriends... The kids left behind can feel left out.. like not getting to go to that birthday party your sibling is attending.

But, that is life.. we don't get to participate or get the treat every time.  

It's ok to want do do something that might even be "selfish"... because if we don't look out for our own interests.. sometimes no one elsse will do that!  

I mean.. my DH brought me a treat home last night.. apple crisp from a favorite place about 3 hours from us.. that he was near running an errand.  He got one of their fantastic steak subs that he ate there... and a few other things..but the apple crisp he brought home for me... so.. I ate most of it (it was a serving.. not a whole crisp..lol).  I mean.. I shared a couple of bites with him.. but I ate most of it.. selfish? well.. maybe a little.. but he could have brought more home if he really wanted to have a bigger piece right..lol?  So.. a small pang of guilt that I didn't split it evenly..but not enough to not let me enjoy it.. haha.

So.. go on your trip.  If anyone gets flak.. he needs to explain that this was basically a gift to your bios that was enabled by your family.. and that they aren't obligated to underwrite his daughter for every trip.. and that there will be other trips for her.. with mom.. with him etc..