See if there is a home where he can live with others with the same needs as he has.
I know your loyalties and your love for him are strong and that is perfectly understandable. But there comes a time when self preservation needs to prevail. Your brother needs special care and help that you just do not have the training or the finances to handle. It is not fair to you to be responsible for him because you do not have the resources to care for an Autistic individual.
Of course you do not love him any less to consider getting him placed in a facility that can actually help him in his journey of life. You are not selfish to want a life for yourself without having to care for another person. Situate your life so that you can still be in close proximity of your brother so you can see him as much as you would like to. Do not feel guilty that due to his special needs, he will do better with professional care. You would not be abandoning him, you would be doing him a huge favor. Yes, it will be very difficult at first, but in your heart, you must know that getting him professional around the clock care is the best thing for him and also for you.
I bet you have sleepless nights worrying about your brother. I can understand that any choice to let him out of your care is agony on your mind and heart. I get it. But, this really is not about you, it is about your brother and what he NEEDS. You will not be abandoning him, you will be doing him a huge favor and an incredible act of love, so he can be the best he can be in a place that qualifies to care for his special needs.
You CAN have the best of both worlds, you know. You can have your relationship with your fiancée and move on to get married and have children if you wish. All the while, during a time when your brother is getting the proper attention and care that he requires. It is the answer to your worries.
Taking the first step is to learn about what services are available to your brother. Please educate yourself and do some research about the benefits that are available through government programs for autistic people who need help. Only 17% of autistic children grow up to be independent. With those low odds of being an autistic independent person, of course there is help available. So, look for it, learn about it and THEN make a choice to try it. By getting help for your brother is the biggest dose of love you can give him.