There are two answers to that...
A technical one and logical one.
Technically and legally: Subletting is when the lessee or tenant rents out all or part of the apartment to another person, while still maintaining status as the prime tenant/shareholder. so, if the person is house-sitting, they would not be paying rent and no legal sublet action is taking place, therefore, no infringement. However, some boards do, in the house rules further define subletting to include housesitting for more than XX weeks.

You need to know first, who is paying the maintenance, and other bills, if it is the house sitter, then let them know that it is considered a sublet (being non-descriptive about your reasoning as possible) and that it could not be permitted; that is, if the Board is opposed to it.
then you might wish to amend your house rules to include other scenarios such as this, time frames, etc.

i typed fast, did I convey the thought correctly?

~AR