Most likely, it is a habit and nothing more. We fall into patterns and often don't realize it. Would you be comfortable talking with her about it if this really bothers you? Parents have so many things on their minds, they cannot be aware of all the things that may be bothering their children, especially when there is more than one to care for.
You could suggest a game-- she'll say goodnight first to the one that is bundled into bed first!
To say mum you say "Mama" (pronounced mah-mah)To say dad you say "Papa" (pronounced pah-pah)To say brother you say "Hermano" (pronounced err-mah-no)
You can say "lafi" to say goodnight in Mandinka.
In Berber, you can say "tanemirt" to say goodnight.
goodnight
goodnight!
"Goodnight".
To say goodnight. and say they love the person
In Fulani, you can say "Jaama wolof" to say goodnight.
In Pangasinan, you can say "napagtolak" to mean goodnight.
In British English, you can say "goodnight" or "night" as a shorter version.
"Shab bekheir" is how you say goodnight in Farsi.
In Polish, you say "dobranoc" to wish someone goodnight.