1. If you are a mathematician, you should choose (b). For the rest of us, (a) and (b) are both fine, but you need to change "who" to "that". 2. (a) and (b) equally good. Jane and Jess are wearing a dress. (Great rhyme!). Again, mathematicians should choose (b). 3. Both sentences confuse me. If you tell me "Jane and Jess have sisters", I would not be sure exactly what it means. It could even mean that Jane has two sisters and Jess has none. To be clear, you would have to say "Jane and Jess each has a sister." Beware, that in currentl lingo "they" sometimes refers to a single person. I don't like it, but that's the way it is.

In all cases, b is correct. By the way, in your first set of examples you can't use 'who' because the relative pronoun represents 'personality' or 'personalities'. A personality is not a person.