This might not be relevant to actual OP, but discussion around siblings has hit a nerve so thought I would share something. I have an older brother with severe special needs. However whilst my needs are not as significant as his, it's worth noting that I also have been diagnosed as having Asperger's and had very significant mental health issues throughout my teens, to the extent I have been a psychiatric inpatient.
I'd also like to point out my experience is deeply coloured by my brother's behaviour towards me in my teens (I heard graphic descriptions of how I would be killed on a daily basis for a few years, I was verbally abused, on several occasions there was physical violence towards me) which has deeply impacted how safe I feel around him.
I think parents - well my parents - often feel a lot of guilt at what a sibling is put through, but rather than constructive ways forward being produced, my experience was my parents would seek my reassurance that I was ok with what was happening, that my brother's issues weren't impacting me. Which produces a weird dynamic in which not only do you not feel permitted to be hurt by things that are hurtful, but you feel responsible for managing your parents feelings about what is happening (I don't really want to go into specifics, but my mum had some very dark conversations with me when I was only 11 or 12 which still haunt me). What I remember feeling the worst about was the precarity of everything, it always felt like walking on eggshells (and still does when I'm with my family), unsure of when the next outburst would be.
I remember in particularly bad times wishing I could become more ill again just to be back in psychiatric hospital, away from it all.
Going to university was an absolute game-changer for me, it finally gave me a space where I felt secure and as having control over my own environment, and it gave me the calm I needed where I could work on my own mental health, and where for once when I was interacting with people, the person they were concerned about was first and foremost me.
I often feel very alienated when discussion around impact on siblings has the implicit assumption that they don't have special needs or issues of their own, or that there aren't instances where their needs should take priority - I remember e.g. my mum on the phone to my brother whilst she was visiting me in psychiatric hospital.
I often felt that my mental health issues ended up mattering much less because they were not as severe, or them not being managed didn't result in violent outbursts (what I did was hurt myself). I am quite emotionally distant from my brother now, and, on one level, I feel guilty, but on the other I just don't have a sense of security that IO feel at ease in his presence.
None of these issues are relevant to OP's discussion, in which it doesn't seem, from information we've been given, that they are being unreasonable in this case, there are no issues of physical endangerment, DD1 and DD2 are similar ages, and the party according to OP is something DD2 can enjoy.