You cant delete an account without moderator assistance (ahem). You may be able to restore a version of your profiles folder from before lil bro got on, like this:
For me, restoring like mentioned before would work. Are you able to do this too? When I am right clicking my profiles folder i am getting this:
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So I could even restore a version of the whole folder from the 9th of july.
.(This is
C:\Users\<username>\Saved Games\Crysis2\Profiles) in Win7
If that works, make lil bro his own singleplayer profile and have him use it when he accesses MP. Each MP character (account) needs their own SP profile to work cleanly. Hope that's at all helpful.