I'm pretty sure both Ian and Jack are variations of John, so Ian isn't a complete unJack!!
Lily-Jack is ok, and for the younger brother Lil' Jack is an option.
But people should just get brave and start calling their kids Jackass, Hijack and Carjack
And for girls there's always Tigerlily, Waterlily or, at a push, Lilyofthevalley.