Down Syndrome Support Group
Down syndrome is characterized by a combination of major and minor abnormalities of body structure and function. Among features present in nearly all cases are impairment of learning and physical growth, and a recognizable facial appearance usually identified at birth. Many other organ systems may be affected as well. If you or your child is suffering from Down syndrome,...

I don't really have any advice for you but know the feeling when no none responds to a request for help. I, myself, am 4 months pregnant and just got the confirmation that our baby boy has DS. I haven't experienced the challenges just yet but know the ups and downs and emotions that come along with it. We already love our boy as I know you love your baby brother but it's always nice to get support and advice. I wanted to send you a message to say go you for stepping up to take care of your little bro and trying to find some answers. I have found a great deal of support from parent bloggers. I'll post some links for you. The first link, I believe, mainly has advice/support for parents/caregivers of children/teens but I bet you might find something for adults as well. The second link definitely has some adult related input. I'll put the links at the bottom. Regardless, hang in there, it will get better :) Take care!!!
https://www.readandspell.com/us/down-syndrome-blogs
https://blog.feedspot.com/down_syndrome_blogs/
My son has down syndrome and they are a handful but they are also a joy. there are groups out there that help with their therapies and such. I don't know what state you are in, but i go through people inc. for a medicaid coordinator for him. She helps get things i need for him, information, therapies. Good for you stepping up to help your brother he is blessed to have you.
I have cancer and now i worry about what will happen to him, if the cancer gets worse, but my children will take care of him, His dad isn't in the picture much and doesn't want my son to live with him. If you want to chat i would love that,.
Good luck, you will be great