OH and I have 3 kids each. OH is in the forces and will be away for 10 months next year - he has his DC's at my house as he can't have them at his base where he lives in the week.
He has no formal access arrangements with either of his kids mothers due to him going away for long periods - his accomadation etc etc etc, but pays both maintenance. He and his DS's mum split up years ago and she is fine about him seeing his DS whenever he wants.
His DD's mum only ever really lets him have them when its convinient for her - meaning whenever he asks to have them (usually a few months in advance) she usually says no although will ask him to have them with a few days notice - knowing he'll drop everything (often plans we have together) as he just wants to see them as much as he can before he goes and doesn't know when he'll be "allowed" 
to see them again. She also messed up our plans at Easter last minute meaning we had to spend a weeks leave apart - and has recently taken to "accidently" sending him suggestive texts which are apparently meant for "someone else" .
OH's DD's live about 400 miles away from his DS. They have not seen each other for over a year because of all the circumstances mentioned above. I live about 150 miles from his DD's. His DS is coming to stay with us for the next two weeks and he has asked several times if he can see his DD's during his leave period and if they can come and stay while he is here so he can have all 3 of his children together in one place and they can all see their siblings - for the first and last time in a couple of years.
She has repeatedly said no due to the fact she says she has friend staying with her for two weeks who has a son (who is an only child) who will get lonely if her girls come away here (we have asked to have them for a weekend or just a couple of days and he will do all the transporting). She has no fixed plans with her friend.
AIBU to feel really angry and sad for my OH and his kids and to feel that she IBU not to let them come for a couple of days to see their brother and to think the reason is quite frankly lame. Surely that is more important than her friends son? Esp as this will be the last chance until the year after next. I feel really really upset for them as the last time they came they thought their big brother would be here and were so disapointed that he wasn't.