Most teenagers will not want a woman to come in and take their dad away from them. My guess is that you will experience a lot more acting out as most teenagers don't know how to express their anger and loss of control over a situation. They didn't choose you and will be forced to accept you as their mom. My suggestion is to wait to marry when all of them are out of the house. Just continue to see your fiance whenever the kids are busy doing other stuff. But I doubt that you will see this as the best thing for those kids. Most don't.
So my next suggestion is to not try to be their mother unless it is asked for. You will know when that happens. For example, the girl might come to you with a request or a conversation about a female issue. One of the boys might need something sewn or repaired. They might ask for specific meals. One of the boys might ask for advice about a girl. But do not give any suggestions or advice if not asked for. Do not try to discipline. If you see something that needs to be done, talk to your husband in private.
It will feel like you are stepping into their space and you are. Let them let you in one at a time, on their terms. If they act out, be understanding and listen, listen, listen. They will not expect any adult to actually listen to them. You don't have to agree but you must let them know that all of their feelings are valid and you can understand where they are coming from.
Encourage your husband to have one-on-one "dates" with each of his kids as well as group outings without you. They will feel left out and they need to know that he cares more about them than you right now.
