I think Elizabeth was a very vain person, she absolutely loved the limelight and the adoration and was very apprehensive of any sort of rivalry.
When she appeared on the scene she was the star, Wallis was completely out of the question, a relationship with her had never been possible above the mistress level. Sister-in-law Mary (Lascelles) was rather dull and as remote as her mother, no other competition for Elizabeth, who was the shining star of the royal family.
Then Marina appeared, and she wasn't only a great beauty, but she was royal to boot. Heavens who wouldn't have hated her?
I don't think they had much to do with another in earlier times, but I have always thought it was a shame the in-laws didn't get on well.
Petty jealousy from the York side and arrogance from the Kent side, I presume.
But when Kent died and left Marina a very young widow with young children alone in a foreign country in war times she paid direly for not standing in a better and more sisterly warm relation to the family.