Oh dear! Joachim is right of course about trying to adapt N. lowii or N. sibuyanensis to windowsill conditions unless your house is so humid that there is condensation on the walls! They need lots of light but direct sunlight through a window in low humidity is death. I have no idea if those plants originally came from my nursery, but it's rather likely considering the species and it's also sad thought to me that all those years of effort taken to produce them may result in extreme highland plants being put on a windowsill. True, I've been paid for them somewhere along the line, but I love these plants. N. lowii will never respond to anything other than extreme care; it's a Specialist's species in every sense of the word. The very, very few people who have ever seen upper pitchers of N. lowii in cultivation will tell you how difficult it has been to achieve such a feat. Jeff Schafer is the only person I know who has managed to do this in basement conditions without the benefit of a greenhouse. Jeff is a meticulous scientist and he would tell you that it takes time, dedication, and yes, unfortunately money too.
N. sibuyanensis is fairly vigorous, but doesn't pitcher well unless it's quite happy with it's surroundings which certainly would not include a dometic windowsill!
Please remember that most Nepenthes are actually quite hard to kill, they hang a in there as best they can waiting for conditions to improve as they sometimes are forced to do in habitat after fire or drought. However, when a juvenile plant (and these are juvenile plants we're talking about) goes into decline, it's awfully hard to turn it around.
Carcinos, if you manage turn N. lowii into a windowsill plant in a temperate country, you'll be making history. Please don't try it unless you are happy to purchase more plants later, or simply don't care!