Just a reminder because I see people just picking and choosing depending on their argument.

- Shadowheart finds Halsin attractive not matter what. (you find out about it mostly through her romance, either party banter or choosing to accept the twins offer with both Shadowheart and Halsin in the party)

- Shadowheart flirts with Halsin no matter what while in a relationship with the MC. (party banter)

- Shadowheart wont sleep with Halsin behind the MCs back or without MCs say so.

- Shadowheart will disapprove if you don't allow Halsin to join in on your foursome/fivesome.

- Shadowheart will mainly focus on Halsin during the fivesome.

- Shadowheart will be open to a poly relationship only with Halsin.

- Halsin will butt in and go for your romance (both Shadowheart and Astarion) no matter what even if you rejected him.

Now, I know a lot of people like to apply the 'out of sight, out of mind'/'I didn't see it so it didn't happen' argument but there is a bit of a problem with that. Shadowhearts attraction/flirting and interest in sleeping with Halsin is a fact, the only thing that is optional/player dependent is the sexual act. (poly/twins scenes)

She was written in a way that as soon as you enter ACT 3 and finish her romance she becomes receptive to Halsins advances. (same with Astarion)
She is meant to have a flirty banter with Halsin and the only thing keeping her from having it is the player not using Halsin in the party. She is meant to have dreams and ideas of sleeping with Halsin and the only thing keeping the player from finding out is not going to the twins with Halsin there.

(I like the majority of players never knew about this in my Shadowheart romance run because for the majority of that run I never had both of them in my party. Like the majority of players I visited the twins only once at the beginning of ACT 3 and got the 'I want to sleep with you first' response from Shadowheart. But what made me aware of the whole situation is that after finishing her romance and at one point adding Halsin to the party the first thing I got as my party spawned in the city was the flirtation. Rest came from YouTube and other forums which I confirmed later on.)