First, congratulations on your impending marriage.
Second, congratulations also on being interested enough in your role as a stepparent to ask questions about what you should — and shouldn’t — do before you get married and are an actual stepparent.
We strongly suggest that everyone who is going to be a new stepparent — or, for that matter, beginning a serious relationship with another person who has children from a previous relationship — read up on their new role as a stepparent. We recommend “StepCoupling” by Albert Bernstein, which is available at your local bookstore or at Amazon (at the following link: http://amzn.to/RpisgF ).
Being a stepparent is hard. You aren’t a parent. And you don’t have anywhere close to the same rights as a parent — either legally, emotionally, psychologically, or in any other way. As a stepparent, it’s usually best to consider yourself as “back-up” to both your new spouse — and to your new spouse’s children’s other parent. It’s not your place to question the other parent, criticize the other parent, or “step into the shoes of” the other parent. And that’s true even if the other parent “isn’t very involved.”
You should consider yourself first a “benevolent stranger.” Your relationship with your fiancee or your new spouse doesn’t entitle you to anything — other than to be another responsible adult in the household, but one who needs to be always aware of your place as a non-parent. Support your new spouse. But also support your new spouse’s former partner. The quickest way to make things bad is to try to take over as a parent “in the absence of the children’s other parent.”
That being said, you ask what are your rights as a stepparent? Pretty much the same as any other nice relative. You can go to parent conferences (but don’t become too involved). Again, be supportive and not commanding, dictating, or insistent that what you want is anything other than a nice suggestion.
Can the other parent stop your involvement? It depends on the nature of your involvement. If you stay in the background and don’t try to become a “third parent,” then there shouldn’t be any reason she would want or need to stop your involvement. If you DO try to become too actively involved, there is plenty the other parent can do to stop your involvement. Again, the children are not your children. And you should play a supporting role (to your new spouse) rather than taking over any kind of primary role. If you follow those guidelines — and the other suggestions in “StepCoupling” — then you should have a great time as a stepparent.