My kids love seeing every single page in my albums, no matter if they "match" or if they're totally different styles (and they're always totally different styles). Scrap how you want and put your albums together in a way that works for you/your audience. I bet your nieces will love what you create for them!
If you want facing pages to coordinate, what about using the same background paper on both? Then you can add as many elements/photos to either page but you'll have that "base" to kind of ground the spread. (This is only a suggestion: you do you and scrap how you want. This is your hobby. If you are like me, then your albums will resemble the scrapbook version of a crazy quilt lol, and they'll look fantastic that way.)
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