Silly me! I just realised I didn't even answer your question.

No I don't think they need a brother or a sister, but it is nice I suppose for them to have a sibling that knows roughly exactly what they are going through in terms of puberty, relationships etc, because they go through it all in different ways, but I think so long as you can encourage a good relationship with them regardless of gender, then they will always have someone to turn to IYKWIM?