Goodness me, I completely disagree that names of siblings have to "go" with each other. I mean it might be a bit odd if one was called Shaniqua and the others Cressida and Mohammed, from a purely cultural point of view, but other than that I don't think it matters in the slightest if they "go" or not.
I've never in my life heard siblings' names and thought "what a shame they don't go". The only time I ever gave it a second thought was with adults on TV when one had been given the grandfather's old-fashioned foreign name and the other a modern American name. I imagined the older one thinking "Why did I get stuck with the weird name?"! 
Marissa and Susannah both sound like nice normal names to me. I know a 55-year-old Marisa so it's certainly not made up.