They are certainly being rude, but can I also say that I see it from the point of view of being a visitor in a country where you don't speak the language and are there for say two weeks. I go to my husband's country (which is near to Turkey as it happens) and after a few visits to see relatives, I often end up retreating to my bedroom/keeping out of the way. It is relentless being on display when you don't speak the language and you aren't part of the culture, I just end up in the bedroom in the evenings as I prefer it there and leave my husband to talk endlessly in his language to his family/parents/friends- I say hi and then go and read. You get fed up of nodding, and there are only so many strained translated conversations (even if they are speaking a little English) before you start giving up and just letting them get on with it.
I wonder if they are staying upstairs to stay out of your way, and also because many people perceive it as rude if they don't speak English when there are English people present- but this again is very tiring. I would invite them downstairs, but make it very clear that they and your husband can speak Turkish freely- we do in our house because we are a bilingual household and always having to default to English and make my IL's speak very broken English (my FIL speaks no english at all) is really unfair in a home context (bit different if guests arrive).
I think much of the advice given assumes a common culture- this clearly isn't the case here, and you allude to them not bonding that well with the children/asking them to do things they don't want to do.
I also think much of the advice is a bit naive as to how hard it is in cross-cultural situations- yes, they could come to the school play or whatever but perhaps they feel quite out of it, unless they speak excellent english. I have noticed this before- English speaking foreign parents who are very integrated, come to everything, trailing parents who don't speak English at all or very well or who are just not going to 'fit' in particularly with the school crowd. Mine are like that- they will come if forced to school events, but it's not particularly easy for them and they don't mostly understand what is going on.
I do get they seem rude and you are frustrated, but I am reminded of the other thread where the IL's (all English) said they were a bit bored in the evenings and instead of everyone slating them, the poster was advised to get some board-games!!! Not on this thread though, everyone just says how rude they are. It is likely that in a 17 day visit they might want a bit of entertaining and perhaps if you were going to Turkey you might also be saying to your husband that you hoped to do some interesting things/get taken out a bit.
This is a long way of saying that given the cultural differences going on, there is a lot more going on here than just some sulky parents sitting in their room. Without more details of how integrated they might be, language abilities, general cultural integration in your family (do your children speak Turkish, spend time in Turkey) I can't really judge whether YABU or not.