Adoptive is the word for a parent (or a family) who has taken a child as their own through adoption — not the parent the child was born to. You almost always meet it in front of a family word: adoptive mother, adoptive father, adoptive parents, sometimes adoptive home. The legal, official flavour is still there, though families themselves may just say Mum and Dad. Though actually, English is fussy about which word sits where: the adult is adoptive; the child is adopted. People mix those two up all the time, and both can be said with love. You would not use adoptive for taking up an idea or a habit; that is just adopt.