I've read through all the replies and think you have got some sound advice.
My younger dsis has ds. Although I believe she is fairly high functioning.i.e. she can wash, dress, cook simple meals, catch a bus etc
My father left us because he couldn't cope with having an imperfect baby. My dm went into protective overdrive and never really stopped!
My dsis lives semi-independently with another special needs friend, their carers come in three times a day to help. Their carers are absolutely amazing. My dsis is invited to all the careers family gatherings, has been taken on holiday with them and has given my mum total peace of mind.
My dsis is one of the happiest, sunniest, loving and giving person you could ever wish to meet. She is extremely content and secure.
However, I grew up resenting all the attention my dsis had. Every family decision revolved around her and her needs/limitations. I feel that I was sidelined somewhat. As another poster said upthread it made me very independent and I ended up leaving home when I was 17. My sister and I are not close, but I love her dearly.
I feel for you as the decisions you are facing are far-reaching and extremely difficult.