
They behave and think like mature 7 years old.
So, I gonna swallow that far-fetched BS of 25 years old.

I know what you mean lol but it was said that they died as teenagers, the sister says that they are adults in their 20s but really they have the mind of teens and while teenagers are mature they can also be immature. So I think how they act really does show that they were kids in their past life and are still kids and fits, but for some reason the sister wants to add to their age (though they technically still have the mind of a teen, not a 23-25 year old). Even the brother said that there is no sense adding to their age when they are kids.

No, in their past lives they were 18 years old when they died (roughly). In this reincarnated life, they're 5 years old only recently----giving them the mentality of 23 year old if one insists on adding up the total amount of years they've lived from both lives like Arien does (18+5=23).
But what's the point in adding those years? Like Arjen and the anonymous user above seem to think: they haven't had the chance to actually grow in maturity levels at all since they died in their first life, given the abuse and isolation in a new world, so it's the same as being stuck at a teenage level of emotional and maybe psychological maturity.
Hey, it's not like I have never encountered such tropes where :
1 - He was never on the potion's influence
2 - Even after getting the antidote, he's still love her
3 - He knew all along and fooled her
4 - He was actually using to get closer to his brother because "what the heck not" - maybe the bro is an expert at snatching his bro's date
5 - He was happy he drank the potion and fall for her. No regrets.
Right all those option are quite what all of us expect to happen. I mean that's how the plot usually goes in those story lines whit love potion involved. I personally believe that it will be something like 1 and 2 together. Like how in the Descendants movie, Ben eat the cookies with the love potion fells in love with Mal, and even after he wasn't enchanted anymore, he still loved her
I just hope the author won't destroy the plot with unnecessary drama and makes us hate the characters
I hope one of those tropes come true and she would still stick to the older brother