That’s a complicated qurstion and kind of outside the scope of the thread. We’re talking about same sex attraction here, not incestuous attraction.
But just so I don’t look like I’m backing out of the argument, I’ll answer it anyway.
First of all it depends on the age of the daughter. We already have a vast array of laws telling us that children cannot do quite a lot of the things that adults can. The reasons for this are obvious - children simply lack the experience to make certain types of choices. But for the sake of argument, lets assume that the daughter is in her early 20s.
Since they are both female, the argument about deformed babies doesn’t apply here. What the couple would have to be careful of is knowing that the love they feel is intimate love, an that they are not confusing it with the type of love that mothers and daughters feel for each other anyway. The only other argument is, what happens when the intimate relationship breaks down? Unlike in non-incestuous relations, the parties are still tied by a family bond. I’m no psycotherapist, but I’d say that would probably throw a faily large spanner into the inner workings of someones wellbeing. But I digress, these things just don’t apply to same sex relations.