I agree with anniebrooke. They sound like really giving people --- and this boy may be your partners. Have a heart. It's just a little boy. And they've clearly bonded with him and trying to stop that or disrupt bonding between your partner and this boy is kind of cruel. And might backfire. you are telling them things about you that are not flattering. You need to not be so insecure that a child that AT THIS POINT is considered his would be in their life or your partners. And I'm sure they'll love your child as much as this boy once he or she is here. But do try what Anniebrooke suggests regarding this other little boy.
It's sad for this other little boy. Remember that. And don't try to be territorial. People have enough love to go around in their hearts.
good luck
Oh, yeah, to the list of throwing a shower, inviting you to live in their home with them, and taking care of a kid that you suspect to be an imposter, I should have added also the buying your princess a bunch of things. I would not be insulted by all of this and look at it as them not respecting you, unless somehow they are implying negative things by their words that are contrary to their deeds, which are speaking positively.
It doesn't sound like you're doing much on the reaching-out end, but are expecting them to do the reaching out, and kind of like you feel dissed by any action they take that is not directly benefiting you and approved in its tone and style by you. It is actually pretty nice of them to throw you a shower, let you move into their house, and to love and care about a little kid that (you suspect) is not your fiance's. They sound like pretty warm-hearted people, or at least ones who are giving the situation the benefit of the doubt and trying hard.
Love is not a zero-sum game, in which people are only given a limited number of love points. It is not as if they spend some on a little kid for the weekend, it means that your upcoming princess will somehow get less, and you are being disrespected to boot. More love comes along with each person who we get to know.
I'd give the situation a chance and work on your graciousness style.