IME a year is no time at all to have moved through the grieving process. It’s 3 years since my mum died and I think I only properly started to deal with the most difficult emotions about 9 months after she’d gone, and the darkest times were definitely throughout the second year.
So don’t be hard on yourself, or think that you’re being somehow too messy or ‘hysterical’ - these are your deeply felt expressions of grief for your dear sister, and there’s no right or wrong way for you to behave. It’s different for everyone in how it manifests and how much time it takes to move through the worst of it.
And talking to people close to you can often feel harder than talking to strangers, so it’s not unnatural that you shy away from discussing your feelings with your parents.
I’m sure you have been a great support for them, but at some point you have to focus on your own grieving. I definitely had to detach from my dad for a bit - focusing on mitigating his pain meant I wasn’t dealing with my own and it made me ill. If the scattering is going to be too much for you, you need to gently step back for your own sake.
Would you and your parents consider separating a tiny portion of your sister’s ashes for you to scatter at a later date when you feel up to it? On the day itself, as people have suggested, you could light a candle and do something small and meaningful for your sister. It might also be a good day to spend alone, try to think about her and let your emotions run free. It’s terrible and painful, but it’s necessary.
If you feel you’re not coping, could you access some help? Cruse are a good resource for support and information.
Eventually I hope you’ll find some peace - you never ‘get over’ it, but you can become more comfortable co-existing with your grief; it can become a quiet background hum rather than something that consumes you 💐