I want to say disrespectful but there are lots of 'ifs' that I have that dont make it quite so bad:
*if it his your sisters 'home' also then some mutual respect between her and your parents has to be found
*if it is a longterm (and by the sounds of it - it is) your parents needs some respect for their relationship
*if this is only a short term thing whilst house is getting built then I do think compromise is needed
I had fairly liberal parents, bfs were allowed to stay over from about the age of 15, in seperate bed/rooms. From about 16-17 onwards (and this is long term bfs, not one night stands/flings) bf could stay in our rooms. Whilst still at school bfs were only allowed to sleepover on weekends.
Atm, I think I have the same views as my parents, but DS is only 18months so who knows what I will be saying when the time comes!