Amusingly, while you're absolutely right that a sister harem is better, Ronnie is hard carrying the 'other choice' with the odds stacked against her. In my perspective. It's a choice between ten characters I've came to be attached to over the course of 5+ years through the show, fan art, and majorly fan fiction. Up against a smaller group of characters I've rarely seen much of (Not so much with Ronnie of course). Sid is such a minor side character to me that I didn't realized there were grumblings about her because I didn't care about the character. Then you got Ronnie's cousin and mother, both of whom I already forgot the names of.
Ronnie is in my eyes, the only thing that the other options has going for. Especially with Cat putting in the work for her character. If anything, it makes me feel sorry for her treatment by the crew over at Nickelodeon. Regardless of my thoughts on Ronnie, I don't wish for her fate to befall on any kind of character.