I feel extremely sorry for him. I know you guys think he's weird and has some sort of serious issues (and he is obviously stupid on some level), but whatever--there are plenty of people like him that support themselves, have friends, and find a partner to eventually have a family with.
His family's business is absolutely ruining him. He once had a certain level of intelligence and was at least average as far as maturity/responsibility due to the burden of his sister. But his parents bribe him to try to keep him part of the vlog so they will still make money. Asa and Priscilla know pretty much how many views he is worth, and how much money he makes them when he's around. They know the show gets less and less watchable and interesting as the three full time cast members age.
Each year that passes the Maasses get less and less interesting, and Abbie jumped the shark years ago. Asa is shrewd. He knows they need young blood and happy young faces on their channel. Their channel "brand" is "smiling lemonade from life's lemons (aka Abbie)." So Isaiah HAS to find a girl and he HAS to get married and he HAS to have kids, all as quickly as possible. Abbie is aging out. These are three full-grown adults, soon to be four, whose entire future rests on some so far unborn child to perk up their channel and provide them twenty years more content. Whether anybody ever articulates this or not, it's the reality.
Yet Asa and Priscilla WILL NOT LET him go out and do what he needs to do right now. He can't so much as have any normal friends or relationships, because whoever it is will be expected to be filmed, and regularly, and put on a good show. Literally not one single normal person would do that, so they've filtered out 100% of any actual good prospects for him right off the bat. If he even found a special somebody among those left over who don't mind performing for free in a circus, his family is going to absolutely RUIN it. It's all well and good that the Maasses, in their 30s, decided to gradually sacrifice all their privacy for money. But there's possibly not one twenty-something female who is willing to join this family, have Abbie as a SIL (knowing what that means--having to take full responsibility for her in adulthood as soon as Big and Bigger pass, spending your holidays with her, probably having to babysit her periodically), participate for free in their creepy family vlog, AND ALSO give up their privacy during young marriage, the honeymoon, being a newlywed, and entering parenthood. If there is even one such woman, I 100% promise you she will not be a normal person or a desirable mate.
So I do feel bad for him. He needs to walk away and put his foot down, but it's complicated. If he ducks out of the vlog, it will cost his family money and possibly jeopardize their ability to continue mooching off their ID daughter. He has a vested interest in them being able to support Abbie AND make enough money to leave some for him to do the same when they pass in the next 20 years. It's indirect blackmail.
I never watch his vlogs because I don't care at all, but he's actually by far the most interesting member of his family. His situation is crazy. He's NEVER had anything close to a normal life and he probably never will.
. I think it’s clear any of the cute factor she may have had going on that Asa was clinging onto with all of his might has well and truly left the building 
. On a serious note I don’t think of this as a bad thing. She’s nearly an adult woman, not a 5 year old child. The humpers and Asa infantilise her and it’s inappropriate. She may be at infant/toddler levels in some aspects of her behaviour and abilities but ultimately she’s still an almost 18 year old woman. The “she’s so cute and too adorable” bullshit comments should have stopped a long time ago. It’s patronising as F towards her and makes me mad on her behalf.
Oooompa Looompa doopity doo…what do you get when you guzzle down treats…eating as much as an elephant eats?
