My girlfriend and I recently decided to be in an open relationship. I've never been the jealous type, and she uses dating for socialization. I'm thinking of asking my best friend/high school sweetheart if she'd like to join our relationship. I know I want a future with these people, and we've even already all talked about eventual coparenting, but she's always been possessive, and I'm worried she'll end up suffering in silence.
If you trust this person enough to want to date her, live with her, and raise kids with her, you ought to trust her enough to be honest with you. If you think that she’s the kind of person who would misrepresent her feelings and pretend to be okay with something she isn’t, do not date her. That’s a serious form of dishonesty and you don’t deserve to be put in a position where you’re constantly trying to second-guess and mind-read someone else.
But ask yourself - where is this concern coming from? Does she have a history of being dishonest with you about her needs, desires, and feelings? Have you seen this pattern in her other friendships or relationships? Or are you just projecting your own worries? It’s generally best not to try and do someone else’s thinking and feeling for them. Why not just expect that, if you ask her something, she’ll give you a straight answer? If she doesn’t want to date you and your girlfriend, one hopes that she would just say so. “I’m worried that if I ask someone to do something, they won’t want to do it, but they’ll say yes anyway” is putting multiple carts before multiple horses.
So my answer is this: if there is a genuine, evidence-based reason that you think she wouldn’t give you an honest answer, don’t bother asking the question. Don’t pursue a relationship with someone you can’t trust in that way. It’s not about protecting her from unhappiness that she won’t protect herself from - it’s about protecting you and your girlfriend from a partner who doesn’t have the skills to be in a healthy relationship.
But if there isn’t a genuine, evidence-based reason, don’t do her the disservice of assuming that her answers won’t be correct. Trust her as an adult with agency that if she says she’s into something, she’s into it. That if she stops enjoying something, she’ll say so. That she can identify and articulate her own needs and desires. And then go from there. If you’re right that she’s “possessive” and would “suffer” in a polyamorous arrangement, she’ll probably just decline when you ask her out, and then none of this is an issue.