All right, if Sr. Bertrille were the ninja (and why not?) who saved two kittens, then she would be "njuna nujna ninja-nuna" or, more properly, according to the rules of Slovene spelling, "njuna nujna nindža-nuna", which doesn't work quite so well.
The aerodynamics wouldn't work and Sister Bertrille would come crashing down.
Or did you mean the kittens were still kittens, only ninja-kittens? Well, that's just silly. But it would still require the dual (not to be confused with the duel that would happen if the ninja-kittens turned on each other).
NOTE: The "their" here (njun- is dual, meaning "belonging to the two of them". Also j is pronounced like the y in yet or boy.
There are a fairly limited number of applications for this phrase, but I can think of one right off the bat:
"When lightweight Sister Bertrille, in her wide-brimmed cornette, rescued the two kittens who had climbed into the high tree branches, the Slovenes all agreed she was 'their necessary nun' (njuna nujna nuna)!"