I was unfamiliar with the stepmom "role" when I married dh, but I had been a daycamp counsler and a favorite aunt and those roles seemed most comforable to me with my stepson, especially at first. Like a camp counsler, or aunt, we played together and had fun, but it was clear I wasn't the mommy. I was a trusted adult who cared for him, like any other close adult relative. I also tried to show him that I supported his relationship with his mom, even though I did not feel it at the time. I would help him write cards to her, take him shopping to buy a present for her, take pictures to send her, hang up pictures of her in his room. Believe me, I was not feeling kindly towards her at all at that time (though, I do now) but I forced myself to do it and it seemed to make him feel less threatened (and she felt less threatened, too, since it was obvious from my handwritting on the envelopes who was sending her these things).
As far as the "your not my mom," thing, it never happened. It got close to happening once when dss was 5. He woke up in a foul mood, and said, "I love my mom. My REAL mom," and I said, "Of course you do, everyone loves their mom." That was my attitude all the time, of course I 'm not your mom, who said I was? Dh had laid down the law before I met him, that dss was to be respectful of all adults who were caring for him-- grandmas, babysitters, day care ladies, etc. so it was just natural that dss was expected to treat me with respect, too.
We've been married for almost 6 years now. I have to say that at first, I wanted the ex to drop off the face of the earth, we were fine without her. But as she has been more a part of our lives (our toddlers just spent the day playing at her house last week, I would have never imagined!!!) dss has felt so much happier and secure. As awful as I thought she was, dss needed her, and to feel like we were ok with her, too.
One warning-- it seems to come up a lot on this board, it is going to be hard for a 5 year old to be away from his mom for so long in the summer, regardless of howmuch he loves his dad. 6 weeks, I can't imagine !