Is grieving for so long normal?

This is a real bummer folks. Just let me get this out. Need to vent. Over the past 12 years I lost two younger sisters. They were not just my sisters, you see. I can honestly say that these two wonderful women were my two best friends. They saw me through a divorce, a child custody issue, drug addiction,loneliness. All the while dealing with thier own problems, and raising children. I'm a 62 year old man. A grandfather. Yet there are times when I break out in these hour long crying fits when I think of my sisters dying. It's been 8 years since the last sister died. Thier birthdays, and Christmas, and Mother's day have become a nightmare to me. I am in therapy (just started) and hoping this will be the answer. But is grieving for so long normal? I am re-married, have wonderful children and grandchildren. I am newly retired but have found fulfillment doing volunteer work. It's just this profound grief that comes upon me suddenly and really kicks my ass. Writing this has helped. I feel better for now. Thanks for listening, whoever you are. Thanks.