With the basic economy ticket, the seat can be assigned either at check-in or at the gate.

If your niece and fiancé check in one second after check-in is possible (24 hours ahead of departure time), they might get lucky and have seats assigned. Then maybe there are a couple seats together to which they can move. At least worth trying.

Arriving early and throwing themselves on the mercy of the gate attendant is the second option. Occasionally one is able and willing to help. Within what they are permitted to do, I've found they actually are quite kind.

Third is eyeballing the seat situation when they get on, and maybe negotiating a trade. Iffy, as middle seats aren't good currency. Somebody with a window or aisle stuck at the very back of the plane would be their best bet.