No worries about being organised, better than letting it catch you on the hoof in twelve months.
It is a bit of a difficult situation you're in there, because caring for someone with Downs is obviously a good direct experience and will be quite relevant in how you approach your career (I know it might be a little insensitive to think of something so serious in such career-centred terms, but bare with me a second). The risk in doing that is coming off as overly-sentimental or self-righteous about the whole thing ("I've got a sibling with xyz, I've suffered more than everyone, you don't know the meaning of hard work etc etc") which is obviously not what the interviewers want to hear or useful in any way. But the fact you've asked this question tells me you've considered this and think you could find the relevant part of the issue and highlight that, which I think should be fine. I mean, its difficult and would take some serious consideration, drafting and editing but it is really up to you and your own judgment.