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.
You don't. If she is trying to mend things then let them be regardless of the outcome. If you are a friend, friends do not date anothers ex especially at this point.
back off itchbay shes mine
Wait for them to break up. If you guys still like each other then, then try out your relationship.
Then shes not your friend if you dont like her
LISTEN TO YUR FRIEND CUZ SHES YUR FRIEND
If your fiend is doing this then she's not your friend. You need to ask her what is wrong. If she likes the guy you're dating then you need to think "Is she going to get over him or will she stay like this?" You need to tell your friend that she means a lot to you but that your boyfriend does too. If she refuses to back off then you need to stop being her friend.
If she didn't say anything back then maybe shes not interested in you, or she good be giving you a run for your money. Trying to see how far you will go to PROVE that you like her.
tell her back off b
ignor him/her back ans said to him/her you not my friend no more or ask him/her why you ignorring me.
yes because shes nasty like that yes because shes nasty like that
You tell he where to go and how to get there. you tell her you really like him and you would appreciate it if she would back off.
shes not sure if she still likes you or shes just being silly or playful trying to be your friend
you have to make her feel comfortable to talk to you shes probably shy so by trying to make her do crazy things or flirt shes to shy to do all that so you have to take it real slow try being her friend first.