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The one thing I would suggest your sister do is to let your parents know "officially" that there will be a wedding. You are supportive, that's great and may be a huge help to her with your parents. I assume the problem is more about his earlier marriage than the age difference?
I would also suggest that she approach telling your parents in an upbeat happy manner. If she is afraid of telling them and her delivery of the news presents her as hesitant and fearful they most likely will react to her negativity. If she is smiling and happy they may see the situation as less negative (maybe not the religious aspect). That aspect will need to be handled in a way that respects your parents view while letting them know it will not affect her decision to marry.
If you suspect your mother knows the announcement will be expected, and she may handle it well especially if she has been quiet about it so far.