Tinni x Ulster (note: is there a bug? Ulster goes to Issach and Tinni goes to Freege in the ending)
Males usually inherit/go to whatever kingdom their parent had, so Ulster goes to Isaach because he is male - and with a male goes the wife. However, with Tinni and Arthur something in particular happens. They are the only characters connected to Freege, Arthur will inherit whatever kingdom his father had (Velthomer, in your case) and so Tinni is left to go to Freege alone because there's no other Freege playable unit to inherit it. No male will accompany Tinni because they never accompany anyone in this game, the wives are the one who do that. The only way to avoid this that I know is to marry Taillte with a non holy blood parent, like, say, Arden. This way, Arthur won't have a father kingdom to go to and he goes to Freege (and his wife comes with him, I believe), leaving Tinni free to go with whoever she marries.
Reminds me when I did Lex x Taillte and paired Johalvier with Tinni, thinking that since Arthur is going to inherit Dozel, Johalvier will be free to go with Tinni. Doesn't work that way like previously stated, the endings are coded based on male inheritance and the male always goes to whatever kingdom he has a holy blood associated, so the only way for Tinni to have a lover in the end is to have a peasant for a father (which I recommend Arden because Wrath x Vantage is still nice even though there's no Paragon, all the other peasant fathers are physical with no benefit to a magical unit like Vantage).
I don't know how this plays with substitute children, if anyone knows and can shed a light on this I'd be thankful.