It is a much better idea to never enter into a romantic relationship of any sort with an adopted sibling, even if this person came into your life later in childhood. The bottom line is that no siblings, whether by blood or adoption, can legally marry—nor should they.

Is it legal to date your sibling?

Romantic and sexual relationships between siblings or other family members is called incest, and if one of the people involved are under the age of consent then it is illegal because its illegal for an adult to date a minor.

Can you date your cousin if your adopted?

Legally, yes, you can marry your adopted cousin, since you two are not blood related. If not marriage, one worries whether they can date their cousin if not related by blood.