She told the Daily Mirror: 'Meghan cannot say she only wants to invite close friends and family and yet exclude the Markles, who have always been there for her.
'It is proper etiquette and tradition to invite family and bad to invite absolute strangers.
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Meghan Markle's half-sister has said it is in 'poor taste' not to invite her to her wedding to Prince Harry - despite being estranged for the past three years.
Out of respect, tradition and humanitarianism, the #Markles should be invited if 2,000 complete strangers are invited.'Our uncle who got her the internship, brother, me, best friend of 30 years Nikki Priddy, nephews.
Fact.'Ms Grant, who is writing a book on Meghan, shared the photo of them together alongside the caption: 'All time favourite pic of her'.
Samantha Grant, who shares a father with Meghan, said she must do the 'morally right thing' and invite her entire family to the Windsor event on May 19.
She said: 'Closeness has nothing to do with it, it is incumbent upon her to do the morally right thing and invite family and demonstrate she has a humanitarian heart'.