I walked in on them having sex?

no do not tell your parents that's the worst thing you can do it would destroy your relationship with all of them. and there only 5 years apart and that legal in tell both are 18 in which age dose not madder ( though i still find creepy white bigger than 5 gap) i would not tell them you saw them id just say you noted that they seem kind of close lately and ask (probably bro rater than mate) what he thinks. and you cant really blame mate for not coming out he my not be ready to admit that to any one even if he has excepted himself enough to act on it. but ultimately you have to relies if they what to be together there nothing you can really do about it. sorry sweety. but keep in mind you care about theme both and dont judge to harshly