Step Families Support Group
Traditionally, a stepfamily is the family one acquires when a parent enters a new marriage, whether the parent was widowed or divorced. This community is focused on many of the relationships and issues that may arise within stepfamilies where you may find answers from some members and you may help other members with their questions.
Although I'm honestly not in your situation yet, I easily could be once I meet Mr. Wonderful.
My best and honest advice, is keep things even keel, simple, and love the two new kids, like they're your own (which I know you do already).
I guess a family meeting, something casual where all the kids have an open forum (respectuflly, of course) to hear the rules/guidelines and yours and Alex's expecations of them. Setting clear cut boundaries that everyone understands is probably what is most important!
You can't take all the load onto your shoulders. With all the kids getting older, don't be afraid to ask for help where you need it. Sometimes we're so set on accomplishing all tasks by ourselves, we let those feelings build up.
You're going to be successful, because you've got your best friend by your side. Let him know how you're feeling, and he'll keep you on track!
God Bless and congratulations again :)
Good luck!