Tags are for what is currently in a game. The lack of a tag does not prove there will not be that content in the future. Unless the dev comes out and says for definite that the girls are too selfish to consider sharing, people can hope.
Realistically speaking, though, which is what this game has always been aiming for, despite all the good humor, and the ridiculousness in the details and character, most strong women don't like sharing. And all De Luca Women are strong women.
I mean, I think we all could agree, that despite all the weirdness in dialogue and all the jokes, and even inhuman abilities of the members of the family, one thing that have stayed true to the real world and have been hitting us all like a bullet train every update is: The emotions and feelings of the characters involved.
In the real world, couple love is not something that people, in general, like to share. It's not about selfishness or selflessness, love is love, and people usually don't like sharing when the love is deep enough. It's why monogamy became a thing in our society. I'm not saying there aren't exceptions, but exceptions are called exceptions for a reason. They are extremely rare, and they require really special circumstances. This game may or may not be one of those exceptions.
And if it's not an exception, it wouldn't be the first game on the site where if you choose to romance multiple girls, you actually end up on the "lonely" ending.
From a writer's perspective, Luna and Gracie MIGHT be open to sharing in the far future if there's some events that offer drastic changes, but in any case, it's quite hard to spin Isabel into the mix. It MIGHT be possible, extremely hard but possible. And I do believe Hopes is capable of it if he wills it, but I don't know if he would. None of us know for sure yet, which is why people have multiple saves just to be prepared.