Between you and I, I think his marriage is in trouble.
I think between you and me is more correct.-
'Between you and I' is an error- it should be 'me'.
My brother is older than I.
'Than' is a conjunction so 'I' is correct'- it's not actually an object and this form would be more likely in formal English. . Many use 'me', though, so both forms are OK.
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