is it acceptable if a man and his elder brother's wife fall in love?

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here is a true story. a man and his elder brother's wife---his sister-in-law fall in love after the brother's death. they want to get married, but it can not be accepted by his mother---an old chinese lady and she is very sad. this kind of thing is not acceptable in chinese traditional conception. what do you think about it? and what is the traditional conception about this in your hometown?