Is it illegal to have a relationship with your foster child?

Well the "child" isn't actually a child but a 17 year old but in 2 months is 18. Can they have a legal relationship when she's an adult?
Update: Im not actually the foster child.
Update 2: Im just speculating! I came across it one day and wondered whether its legal! Oh and he fostered her when she was 17.
Update 3: And the foster father is 23
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