How do you tell your friend you like her ex that she's trying to get back?

It's important to realize that your sister most likely will not be ok with you dating her ex (most girls are not ok with stuff like this.) You have a choice to make: what is more important to you, hurting your sister and potentially damaging your relationship beyond repair, or pursuing your sister's ex.

Do you think this person is "the one?"

Do you think your sister's ex has feelings for you too?

If you answered "no" or "I don't know" to either of those questions, maybe it's best that you let this one go. There are plenty of fish in the sea.

However if you simply can not endure another day without this person in your life, then be honest with her. But do it carefully. Here are some suggestions to help you avoid a conflict:

Keep in mind how long were they together vs how long they have been apart.

They say it takes half the time spent together to get over somebody so keep that in mind when you decide to tell her. If they have been together for 2 years than its probably best to wait until they have been apart for at least a year.

Feel her out:

Before you say something you can't take back, try feeling your sister out to see how she reacts to you causally mentioning her ex. Maybe "find" an old photo that he is in as an excuse to bring him up.

Also, saying something complimentary about him will most likely prompt her to reveal her feelings of hurt, animosity, or indifference. Try "he was funny, do you guys still talk?" something along those lines...

Once you have established that she is not still hung up on her ex, ask her if she would be ok with you dating him or hanging out with him but keep your tone casual. Again watch her reaction, if she is very angry or taken aback you can change it into a joke and say it's been a while and you are out of leftovers and chuckle... (something along those lines) this will make it seem like you are kidding around and you can avoid upsetting or angering her.

If she does not seem angry (note: she will probably be surprised) then you can proceed in telling her that you like him. Be honest and respectful of her feelings and hope that she is understanding of yours.

There is no guarantee that she will be ok with this, so pay attention to her reaction and decide if you want to go through with telling her.