1. If they are kept completely out of natural light, at a constant temperature year round then they won't know its winter.
2. Cool and dry is best for most adult cacti. Young seedlings generally don't have the reserves to last several months without water. Also, they will soon grow out of any etiolation from the warm but dim winter, and it usually isn't relevant to try and set them up for flowering by letting them go dormant for a while.
3. Plants perched on a windowsill almost certainly will know the difference between summer and winter since their major source of light, the sun, will vary both in intensity, colour, and timing. They may also be exposed to cooler conditions, certainly not 80F+ days and possibly chilly nights next to the window. But, ignoring that minor problem, they will continue to grow if they are kept warm enough and most indoor rooms will be warm enough. Lack of strong sun will cause them to grow more weakly, as in thinner spines and taller thinner stems, and they will need less water because of the weak sun. Some species may fail to flower without a cool dormancy.

Shrubs n Bulbs, thank you for your help.