I think I got it.
Ashley: Hey Tali.
Tali: Hi Ashley, is there any problem?
Ashley: I'm fine, just checking how's it and engine room.
Tali: Well, everything is fine.
...
Tali: I don't think that's the main reason you're there.
Ashley: And you would be right.
Tali: *sigh* This is about thing I have with Shepard. I know you were together before, but I love him. I would like to stay friend with you, but I don't want this be a problem.
Ashley: Don't worry, it won't.
Tali: Really?
Ashley: Yeah. In fact I wanted to tell you I give you my blessings and to say for you two to have a happy relationship.
Tali: I- I didn't expected this. Because I remember you were quite riled up when we Liara joined our crew.
Ashley: Oh, that. Well, I didn't know Liara at the time.
Tali: I know, that mysterious women could have been quite the competition for Shepard. That would have spark the jealousy.
Ashley: I wasn't... Hmh, who am I fooling?
Tali: Well it was fun seeing you like that.
Ashley: To you maybe.
...
Tali: Though why is it different with me.
Ashley: Well, we are friends. More then that, in fact.
Tali: What do you mean.
Ashley: Well, you're kinda like my little sister. Part of my family. Kinda silly I guess.
Tali: *chuckles* I never had a sister.
Ashley: Yeah, due to preserving resources for Migrant Fleet.
Tali: You know, I would like to have a sister like you.
Ashley: That's nice of you.
Tali: But I mean it. You always did treated me well and I can now say it treated me like your sister.
Ashley: Well, that may have happen.
Tali: Don't be modest, big sister.
Ashley: *chuckles* Well, I guess I won't little sis.
...
Ashley: Oh and, if Shepard ever does something wrong to you. Call me.
Tali: If he does it, my shotgun is always right next to me. Plus, it would be quite the overkill if you join up.
What do you think?