"Take care of the little one(s)" could cause one of two things:  Either the empowerment or rejection of the eldest.

If the first born feels important for helping, but does not lord it over the siblings, good for everyone.

If the first born feels important for helping and becomes a tyrant, bad for everyone.

If the first born feels unimportant because the replacement(s) are more important, but they try to include the eldest in their activites, good for everyone.

If the first born feels unimportant because the replacement(s) are more important, and they agree, good for the replacements.

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How it was said and administered would make most of the difference, unless there was a "wiring" problem in the eldest.  What the little one(s) did with it would make the rest of the difference.