Catholic

“We have already planned to marry at our reception location. We will not be using the church.” End of story. If she tries to push back tell her the decision has been made and it isn’t up for discussion.

I'd simply tell her you don't want to get married at the church, there's really no way around this one unfortunately.
It might come across better for your husband to tell her and to mention you both wanted ceremony and reception in the same location.

Talk to a priest. Ask him to explain things to your mother. They will not allow two people, whether baptized and raised Catholic or not, to marry in a Catholic church, when neither is now, nor intends to be, a practicing Catholic. My FI's parents, and parents of several Catholic or Lutheran friends, have raised a fuss over them not having a Catholic Church wedding, or one in their type Lutheran church. But stopped interfering with the grown kids weddings when told by the church priest or minister, sorry, but we won't marry them in the church even if they ask. Bot require extensive premarital preparation in the church, and that at least one be a current and future active attendee of a Catholic or Lutheran church. . . To get you booked, mother must have misrepresented things, stating you had not attended much recently but wanted to take pre-cana instruction, and fully intended for the one raised Catholic , or both, to start and carry on in the future as a practicing Catholic. Let a priest break the news. Once she knows a Catholic Ceremony is out of the question, you need to take back your wedding planning. With any luck the priest will admonish mom, as his priest did my FFIL. He told him that true faith in the Chuch and all it represented was fully a matter for the couple, and that parents should not interfere with the religious practices of their adult children. The priest will nit himself, or allow others, to " stand before God and lie about belief in church doctrine, or their intent to fully practice the Catholic faith in the future." Stop their messing with the religious content, or lack of it, by letting clergy they trust do it for you. A generation ago, priest mostly allowed it, accepted just promises to convert, or practice the faith. Not anymore. Now they will postpone your wedding until you have met all their standards. Marriage is a sacrament in the Catholic faith.

I would suggest having FH do it, just because I am a believer in the "he deals with his family, you deal with yours" method of working out issues. Your FH should tell her that you won't be getting married at the church, and that if she doesn't call and cancel by x date, that he will call himself and cancel. He needs to tell her that this is non-negotiable and there will be no discussion about it. If she tries to argue or resist, he should walk away from the conversation and not entertain her in any way, and then just go ahead and cancel it. This is a serious overstep of boundaries, I hope everything works out.

I agree with what the PPs have already said, but also note that your wedding is in January and it seems your FMIL's concern is inclement weather. This leads me to believe you plan to have an outdoor wedding otherwise why would her niece's wedding getting rained out bother her for your wedding? If you are planning to have an outdoor wedding, then make sure the weather is mild enough for your guests to tolerate sitting outside for at least an hour and make sure you have a backup indoor plan in case of rain. If you have those things figured out then it might alleviate your FMIL's concern.

Wow. Not only is she trying to force you into a venue, but a religion too. What a sensitive subject. Can you all get together for dinner and have a polite but firm conversation with her. Let her know how the planning is going, and that the venue you've chosen is the venue you will be wed at. Hopefully she understands. Thank her for her help, but let her know it isn't needed and that if you do need anything you'll be happy to let her know with future planning. All the decisions are yours to make, be appreciative but firm.

I think the priest can take care of this for you. Unless your FMIL found the chillest priest ever, they will not allow you to get married in the church without jumping through a bunch of hoops, especially if you weren’t baptized in the Catholic church. I’d be honest with him and more likely than not he just won’t allow you to be married there.

View Quoted Comment It is outdoor. But we live in south Louisiana! They also have an option for indoor if weather is severe!

View Quoted Comment Yes no full mass and a deacon!!! I was baptized catholic and she is clinging on to that.

Just let FMIL know (with FH) that you already planned you ceremony location. It was really presumptuous of her to book a church without talking to you both first. You don’t have to let it get into a religious debate; keep control of the conversation, and let her know you two already have it covered!

View Quoted Comment As long as you have a contingency plan for weather, I think you're set. Maybe present the contingency plan to her and let her know you have it covered. Maybe she'll take the hint.