They are 2 boys, aged 6 and 10.
She moved when they separated about 4 1/2 years ago and now she just says they're busy when my brother emails to arrange a date. She's never agreed to arranging visiting dates in the future (apart from for the holidays), so it always has to be done 1 visit at a time, but this means now that she says they're busy or the children have a sleepover etc.. that it can become nearly 6-8 weeks between his visits.
They split the holidays fairly equally, so I don't think longer visits in the holidays would work. I think the issue is more that he likes the regular contact in term time in order to ensure his relationship with the boys doesn't deteriorate.
Thanks Clare - i'll forward that on to him.