I wouldn’t worry so much about what being jealous of your fiancé’s daughter says about you; your relationship, which you believed was between two people, has suddenly, without any warning, expanded to include three people and it’s natural you would experience some growing pains. What I would worry more about is the idea that you don’t seem at all interested in growing into your new three-person family relationship. Like it or not, this little girl is going to be a part of your life forever if you marry her father, and the time to “fit in” with her is not after you get married; it’s now. Waiting until you’re legally hitched before maybe forging a relationship with your stepdaughter is like waiting until after you get knocked up to consider your birth control options.
Your fiancé and his daughter are a packaged deal now. For whatever reason, you didn’t seem to understand that when you said ‘yes’ to marriage. Although it’s strange that you may not have even considered that a man’s daughter might be part of the picture eventually, I’ll give you the benefit of the doubt and assume that your fiancé didn’t make explicitly clear that he had a daughter, which in itself is kind of shady. But now that you know that he does and you know he’s going to do everything he can to be part of her life forever, you need to alter your idea of what you thought your marriage was going to be like. Before you make any wedding plans, you need to live with this new idea of your marriage — one that now includes a four year-old — and see how it sits with you. You aren’t a bad person if you decide it isn’t for you; but you would be at fault if you decide it’s not for you and you get married anyway.
This is the time to test the waters. The jealousy you feel is natural and human, but you need to push it to the side and try to embrace this child. If your fiancé is down in the dumps around you it’s probably because you aren’t encouraging his involvement with his daughter and you aren’t showing any interest in getting involved yourself. Can you imagine how he must feel being engaged to a woman who doesn’t give a lick about his kid? Who isn’t excited for him that he’s finally been reunited with her while she’s young and they still have most of her childhood ahead? A true supportive partner would put her own feelings of jealousy aside and be excited for him! If you can’t find it in your heart to do that — to at least fake it until you feel it — you should do all three of you a favor and MOA. Marriage can be hard, in large part because we often have to put our own feelings and desires aside for the good of our partner and partnership. If you already know you have trouble doing that and aren’t interested in changing, it’s probably wise — and kind — to bow out now.
*If you have a relationship/dating question I can help answer, send me your letters at firstname.lastname@example.org.