The Donald might have given Miss USA Tara Conner a reprieve, but he might not be through with her "little sister" -- and rumored hookup -- Miss Teen USA Katie Blair.

An inside source tells TMZ that Donald Trump is considering taking action against the 18-year-old Blair for her "improper behavior" with Miss USA, which has allegedly included underage drinking and hooking up with guys -- and Conner as well. We're told that he's paying extra-special attention to Blair.

Trump, according to our source, is also on the warpath to find out what's not being disclosed by pageant contestants in the wake of Conner's saga and the stripping of Miss Nevada Katie Rees' crown after racy photos surfaced this week. We're told that the Donald wants to know what these girls are hiding -- before they're allowed in the pageant.