I am a guy but I believe many women enjoy the 'sensitivity' of a guy who is ticklish and especially if they admit it. It's such a cultural thing to be the tough manly guy that when one's ticklishness comes out whether in a gentle, playful situation, (or even if you're being wrecked by a dedicated female LER)...I believe that women find that 'weakness' endearing...either because we let it show or they get to expose it by tickling us to bits....
and on the older sister comment...yeah...Im betting they did really enjoy it...being in control of a guy (any guy) and not being the one automatically held down and tickled (I mean the predominant scenario is M/F in all of recorded history)....
I know a female LER to me is the same feeling when I meet a woman who is an active duty LEO or federal agent....Or even just shoots regularly, plays golf cause they like it....rides a motorcycle ...then there is the female plumber or mechanic .... sign me up!
now I dont know if the guy acts like a goofy giggly high schooler when tickled...but who knows?