You can love your mom, but it'd be weird to be "in love" with her, no?
"In love," as best as I can tell, is deep romantic love, as opposed to a strong caring for someone like the love you have for your mom, sister, brother, or dad.
Also, When you are IN love with soemone you care about them deeply to the point where you don't even want to consider being with anyone else at all. (in romantic terms that is)
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