Thank you for your question and I look forward to assisting you. That being said...
In my opinion, you need to nip this bad behavior on fiancee's part, and that of his parents, in the bud. You are the mother, period, end of story. And their desire to keep you away while they take a visit is horrendous! And if you are thinking of marrying this man, I would think twice - he is clearly completely disrespecting you and also not a partner. Under these circumstances, when you have adverse extended family members seeking to separate you from your child so that they can clearly NOT stick to her diet (breastmilk is absolutely, undisputedly among experts, the best thing for your child AND understood by the Courts to be such) - and lest face it, they don't like you or respect you, and I suspect your fiance is partially, at least, to blame. I would play hardball - I would not let them see my child without me, and given their poor attitude, I would likely welcome them for a brief visit to my home but I would not even want to go to theirs. If they want to see the child, allow them to visit :)
But this is actually more frightening than that. I sense that they are hoping you will not be marrying their son and given their complete disrespect for your parentage (and your fiancee's ridiculous attitude), I would start protecting myself for future custodial battle - the writing is on the wall. And your fiance clearly hops to their "jump". I would not share this new knowledge with the fiancee! You will lose your upper hand - your upper hand right now is your ability to accumulate evidence without him hiding it. First, make sure to continue breastfeeding, bottle or other. Make sure to be unable to accumulate any great excess (i.e you don't want tons in the freezer that will last days or weeks if you are can't be with your child) - make sure that you are simply not able to produce enough to "get ahead'. DO NOT share this with anyone. My thought here (incidentally I too have a 4 month old, soon to be 5 months, but he is a boy, and I too pumped) is to make sure you have the legal upper hand for 1-2 years at least with regard to not being forced to be away from your very young child more than a couple of hours...this way you can likely win any legal battle in that regard. Woman, you have total say, so DON'T hand that over to the father or his parents, god forbid. And you really have to get rid of the father in terms of marrying him - he is so NOT on your team, and I dare say (after discussing this with my male legal colleague) that he, given his threats in particular, is looking to fight for custody, knowing you two will break up. But you have the opportunity to prepare in advance for this likelihood. Take it! And show him the abundance of documentatin showing the benefits of breastfeeding for a year, to buy you time to NOT show your cards while not allowing your child to be out of your custody. My fear is also that he may not return her, and then you have a battle. I also recommend, under these crazy circumstances, that you consult with a family attorney/custody in your locality (but not so local that the secretary who knows you might let it slip that you were in the office), on the sly, for an hour or so to get some input. I feel strongly you need to be proactively protective of yourself and your daughter, not that your fiance (ugh) is a danger, but because I sense he will seek to take her via custody.
Hope this helps to clarify.
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