Although you may feel as if telling her she is fine is the right thing to do, it will most likely lead her to feel worse in the long run. There are several reasons for this.
There are two types of reward systems at work here: short-term rewards and long-term rewards. Each time your niece calls you, she is seeking to feel better right away, this is a short-term reward.
Think about a grocery store. They put that candy right at the checkout aisle, and you and your child have to stand next to it, look at it, and smell it. If your child asks for candy, and you say, “No,” your child may start to scream and cry and throw a tantrum. If you tell her that she can have the candy if she just stops crying, you have just taught her a way to guarantee getting candy (short-term reward), throwing a tantrum.
The chances of her having a tantrum the next time you are near the candy have now increased. Or, you could have let her cry and scream while you checked out, and then left the store, teaching her that throwing a tantrum will get her nowhere and therefore decreasing the chance of a tantrum in the future.
Similarly, your niece received a short-term reward from you when you told her that she will be fine. She will now be more likely to want that reward again because it relieved her anxiety so quickly. This makes sense, why wouldn’t people want quick relief? Yet this is also the problem. Because that quick relief feels so good, she will not face her anxiety and learn to handle it in the long term. Also, every time you tell her that she is fine, she feels that she is no longer responsible for anything bad happening, you are. Because you gave her reassurance about her health, you are the one who is now responsible for her health, if something bad does happen to her, she is going to blame you for it, thinking you lied to her.
What will most likely happen is she will call more and more, maybe up to one hundred times a day. Therefore, as difficult as it may be, you need to start to decrease your role in her OCD. You can cut your reassurance-giving down to every other call, and then every third call, and so on, until you finally tell her that you are no longer going to answer her questions because your reassurance is helping her to actually maintain her OCD instead of decreasing its effects on her life. If she protests, it may be a good time to suggest some professional help.
And, of course, if she is involved in at-risk behavior that could lead to HIV, she should be tested and seek help immediately. However, most people with OCD are not involved in these behaviors, yet they still fear the chance of getting the disease.