I think they certainly loved each other, perhaps in a ‘simper’ way than Jackie and Jack loved each other or Ted and Joan did. Many people who knew Ethel and Bobby said they seemed more like siblings than husband and wife; they were unconditionally loyal to one another, despite their individual faults and flaws, were the best of friends, and loved each other deeply. From what I’ve gathered, they complimented each other and brought out their contradictory traits in one another, thus making them better balanced people, and making their relationship all the more stronger: a large part of their individual personalities is that Ethel is very competitive and craves chaos, while Bobby was more gentle in nature and grew up in and therefore yearned for structure. Ethel is a ideal political wife, with political potential herself; she is driven, loves power and found the ideal medium between superficiality and realism that the public admires in a politician. Bobby never wanted to be Attorney General; he wanted to move to Boston and become a lawyer and lead a simple life with his family, but Ethel didn’t want this, and essentially brought out the leadership qualities in him, that drove him to run for the presidency in 1968. Alternatively, Bobby was the more nurturing parent of the two, and brought out those traits in Ethel. Also, it’s no secret that the Kennedy boys weren’t always faithful to their wives, and although Bobby perhaps felt worse about his affairs, Ethel tolerated it not only because of her unconditional devotion but because she loved nothing more than being a Kennedy wife (and all that the title entailed).
Sorry, this is a terribly put-together paragraph, but I think it sums up my interpretation of the relationship.