Perhaps he's not autistic but has Aspergers? If he's nearly 30, it's likely that simply having a normal IQ will have "excluded" autism and no other diagnosis made, whereas nowadays services are more aware of autistic features in the normal/average population.
It sounds like life has been difficult for him and your DH is sensitive to that and very caring. Maybe try to see the positive in that. Presumably you love your DH and the kindness in him caring for his brother on this way is part of who he is and why you love him?
BUT the behaviour you are tolerating is unacceptable. I think you need to tackle it directly and frankly. You can be kind but clear.
For example:
Gross comments about breast feeding "Breast feeding is not gross. That's a rude thing to say. As this is my home, I will be breast feeding where I am most comfortable. You will have to go to another room if you want to avoid this"
Taking over the living room with gaming "we are playing in this room now, the game will have to stop now as it isn't appropriate around children"
Just keep stating what needs to happen calmly, kindly and with brief explanation. You can just repeat yourself if given argument. The bottom line is this is your home and you have rules -and you may need to state that.
It would be much kinder to be calm, clear and assertive as things come up, to try to make this work, rather than accommodating him until you reach your limit and lose the rag.