I disagree with you, first a relation with a guy like Mark will put way too much strain on their relationship over the years and who do you think she will turn to for support and comfort when the going gets rough?
Moreover as I said before as she is written Nikki for some reason in the past Nikki bonded to Tales as her partner emotionally at an important age, instead of what normally happens that siblings create a space between themselves to create room for future partners. Now whether that was after her first bad sex experience or that this was only a reconfirmation that Tales is the only man she could fully trust who knows. Now her behavior shows she has been fighting against that bond and relationship and not accepting it. Well unfortunately those very real bonds and deep emotions, are just as real as your right hand or your stomach. They do not go away just because you say No, that takes acceptance, dealing with the reasons and underlying emotions why you bound yourself in the first place and then a lot of willpower and really wishing to change those bonds. Likely there is no real happy ending possible for Nikki after she bound herself as a partner to Tales years ago for whatever reason she did. Tales did not really reject Nikki either though, he just wanted clarity about what their relation would be about. Besides his feelings for Nikki run pretty deep as well, even if he would not see her as his partner first. Really refusing her and her needs would be extremely hard for him even if he really wished to.
Bit more stretched conjecture, though it seems Tales was way more successful at creating space between himself and his sister at puberty, even on his side there are signs that at least part of his staying in his shell and not looking for a girlfriend may well have been partly for the fact that Nikki was already taking care of all his emotional needs you normally look for in a girlfriend except for the sex part. Now under those circumstances I see no real option for her to have a long and happy relationship with Mark and that is a very sad ending for me. You are free to disagree with me though.