If you don't live near family, how long between visits

[niccig]

My parents live in Australia, I live in USA, 1 sister lives in UK and the other sister splits time between UK and USA.

For the last few years, my parents won't visit any of us. I think they feel that we must go visit them as we moved overseas. They've never expressed this, though I know my mother shuts down to not deal with emotions and it is difficult to visit and then leave. They're early-mid 60's and still capable of travel, even though I know those flights are awful. They can afford to travel, unless they spent all the money my mother inherited (a big chunk of change). They're both retired for over 10 years.

We visited them the last 3 times. Last trip was Dec 2010. We're not going back this Summer, so it's been 18 months. We told them in January and they said they'll think about coming to visit in July. I haven't heard a peep about it. I told DH we'll have to go next Summer if they don't visit as then it will be 2.5 years since we've seen them. 2.5 years!!!!!

I don't know what to do anymore. They don't call us either. I called in early April for Dad's birthday and will call this weekend for Mother's day. 4 weeks and no phone calls from them. I didn't call to see how long they would go. I know they're doing this to my sisters as well, so it's not just me.

I see my sisters either every 12-18 months, usually here in USA as they both come for work or one comes to ski, so we travel to meet them. My parents have been invited to all of these gatherings, but have always said no.