YABU. I’ve no doubt that she has her faults, and hasn’t always behaved perfectly (to what extent, is less clear to me), but I don’t think that makes your actions acceptable either. I think she’s probably not quite the ogre you’d like everyone to think she is and you aren’t exactly the nicest step-mother either.
The bottom line is that your DSD deserves to be a part of her father’s life and to visit him if they both want that. If you cannot accept your DSD as a small part of your life (and it is a small one if she is only physically present for a few days a year) then that’s completely on you. I suspect you won’t make any changes, though, because it’s easier for you to allow her to be the villain than accept you all have your faults.
After all, you CHOSE to marry her father, in the full knowledge that she either bullied your daughters or was friends with those who did. If you could not stomach that, or her, you should not have continued the relationship (regardless of your daughter’s saying they’d be ok with it). But you did marry him, much to you and your children’s material gain, so now you have to either find a way to co-exist for the brief periods she is around, or temporarily remove yourself from the situation when she visits her dad. Or move on permanently.
If you genuinely love your husband, you would never seek to place a wedge between him and his daughter, however you might personally feel about her. Making her feel unwelcome in her own childhood home (one that you have moved into with the adult children who make it clear that they resent her presence there) and even asking him to support you in that, is an absolutely awful thing to do. Quite similar to bullying really, isn’t it? So, you are no better than the worst thing you think she might have done!
Did you actually expect your DH to agree to his daughter staying in a hotel rather than in his home because you and your daughter’s don’t like her? I think perhaps it’s not only the DSD who might have fooled those around them when it comes to their “true natures”.
As for your DSD, I am not clear whether she did or did not bully your daughter(s). Maybe your daughter is 100% accurate about her- but if you honestly believe this, why did you not address this in some way before making a permanent commitment to her father? Why did you as parents not try to get everyone to move forward in some way? Not necessarily all is forgiven, more that we accept that in this situation we all have to be able to comfortably tolerate each other if this is going to work? And if your daughter’s genuinely were not able (or willing) to do that, why would you marry her dad? How did you think that would possibly work?
I think that both your DSD and your DD could have their reasons for lying about the situation. Or perhaps neither your DD nor your DSD are intentionally lying, they just have different interpretations of those past events- the truth is could possibly be somewhere in the middle. For example, DSD friends with the bullies, perhaps not actively engaged in instigating anything/name calling etc but not trying to stop them either and she did join in the sniggering/laughing at something the bullies said or did on occasion. Not nice behaviour at all, though many of us will have done this at least once in our teenage years- I know I definitely have (not proud of it, but I don’t think it made me a major bully either who deserves ongoing censure). So, perhaps not as innocent as she is stating but not as bad as your DD has made out. Certainly not something to be punished for forevermore by being pushed out of her father’s life as far as possible. Your daughter(s) probably know that she wasn’t the instigator but (understandably) resent everyone involved. Your DD’s probably do dislike being around her because of the past and also perhaps there is some satisfaction at being able to confront/get at someone. Being autistic doesn’t mean your daughter can’t also get things wrong or do unkind things on occasion.
I’d also say to you- guilty or not, at what point is your DSD going to stop being unwelcome to you when it comes to an active place in her father’s life? At what point will she have sufficiently atoned for her adolescent/ young adult sins for you to decide to stop attempting to shut out of her father’s life as far as is possible, in order to make your lives easier?
I’d also suggest to you that she may not have found your family’s entry into her life pleasant or easy? She may not have been totally reconciled to your relationship with her dad (he has a right to a relationship, I’m not saying she is right to feel that way)- and I think you might reflect that this situation may have been hard for her too, you know? You and your children moved into the home she grew up in, moved in with her father and she may have felt a bit outnumbered- especially if you and your daughter’s were somewhat hostile towards her (overtly or not). She may have felt pushed out of her dad’s life to an extent, in favour of you and your family (however wrongly) and lashed out at times.
I could be wrong, but you mentioned an inheritance paying for the house and it sounded like her mum may have passed away (reading between the lines) and if that is the case, I think it is not hard to see why she might not have been thrilled with the situation- especially if she sees her dad massively financially (and possibly emotionally) supporting your children whilst they and you actively resent her presence in her own home/father’s life. She may feel pushed out by you all moving in- or feel that her dad has started to put your DD’s needs first. Perhaps, if viewed in that light, a few outbursts and snide comments are unwarranted but not inexplicable or unforgivable. And the examples you have given of her being unkind were some time ago- nothing recent mentioned. If that is the case, I think you do need to leave it in the past, especially as she apologised at the time.
Her comments towards you daughter about sponging off her dad and needing to grow up are unkind- but she is right that her father is funding your daughters’ lifestyles to a far greater extent than you are (and presumably far more than their actual father)- with no end in sight- maybe that worries her? It does sound like he gives them substantial support, more than you could have done without him, and perhaps more would be usual for the spouse who came into adult step-children’s lives when they were very late teen/young adults. Ultimately, that’s your DH’s decision as it’s his money to do with as he wishes, and your DSD’s issue to come to terms with, but I’m not surprised she isn’t thrilled by it. From her perspective, it could look like you’re a bit of a gold-digger who is trying to push her out for the benefit if you and your children (and if any of that may have come in the form of an inheritance due to the loss of her family- mum or even grandparent, for example, then it would sting even more. She may not be correct, but her prejudices and her assumptions about you are no worse than your attitude towards her.