My Mom is becoming more and more controlling over our Dad and their 'privacy'. Any advice?

My father really wants to see his children and grandchildren.They live on their own, in a rural setting. Their frailty is advancing though they have no plans to take on some outside care. One sister living nearby is becoming more and more taxed with little incidents. What is the 'bunker' mentality my mother is developing? How to approach her without getting her defenses up? She claims illness (not the case - but she is a drama queen even for a cold), or stress with too much family around. We have never been overbearing, and in fact everyone is scattered all around so when siblings make plans in advance to visit for holidays and she pulls the plug, it is really disruptive. She cancelled Thanksgiving and Christmas this year. My father goes along with it to keep the peace, and we figure he has to live with his choices as well. But it is heartbreaking to hear his voice wishing we could have come...