If older kids are being babied...
 

...how can it work without coming off as child abuse or negligence parenting?

Like, I want diapers and bottles to be involved during home life, but the idea is that the twins my story follows have to balance growing up on one side with a baby side. So I was thinking that despite being out-and-out babies, they knew how act like big kids.

The question is what would make their caretakers agree to them wearing diapers and sleeping in a crib?

First thoughts coming to mind were either reality-warping magic or some psychological trauma. With the fact that I like my stories being more upbeat has me leaning toward the former.

I need advice in regard to this.

Well , could be just complying with the child's wishes? This is fiction we're talking about so you don't need to abide too closely to what can realistically happen. You can maybe have the parents believe in a more secure psychological upbringing , making sure they get the most out of the early stage of life. If you just try and think of an idea that's not benefiting the parent then it's less of an abuse story, you can go the medical route , as in the kids need the diapers or need a bit more care being in the form of cribs and bottles.