Nothing at all wrong with biggest, or bigger or big, in the right context.
Yes, it should be the younger of two, but I think most of us may have referred to our youngest child (of two) - at least the meaning is clear.
Some strictly correct forms actually sound wrong to me - 'It is I' for example. I think I would always say 'It's me', although I know full well that the verb 'to be' takes a nominative complement. My goodness, how those phrases learned nearly 60 year ago stay in your mind.
On the other hand, 'I got it' is quite correct but sounds clumsy to me - well, at least it is not 'gotten'.
I enjoy the niceties of grammar - I use to love parsing and analysing a sentence - now I have seen teachers on quiz shows who didn't know an adverb from a pronoun.