@JanineR I'm sorry, I just don't think your later explanations are quite what happened. You gave a very different impression in your first few posts, and it is hard to understand that as you could only have two witnesses, you didn't each choose to have your own Mums' and either leave it like that, or have the Aunt as well because you are both close to her.
If signing the paperwork at the Registry office doesn't seem important to either of you, then why not live together until you can have the Roman Catholic Wedding that is obviously so important to - which is fine in case I sound disapproving?
However, did your DFiancé want to join the RC Church before he met you, or is it a case of if he wants to marry you then he has to be baptised into your faith, and presumably he has had to agree to your children being brought up in that faith, which will presumably also mean that any Christenings, confirmations, etc. will be led by your, and your Mum's beliefs? As I said before, all of that is fine because your DFiancé is am adult, so is presumably agreeable to all of those conditions.
But what it does probably show is that you have already got your own way in a lot of things to do with this Wedding, and that it would therefore be kinder and more generous of you to let your DF have more priority over the guest list at the Civil Ceremony, as that will presumably be of more importance to his family members, than to your DM and DSis? I also expect that your family members will give much more importance to the religious ceremony, than the civil one. Of course I am making assumptions here, and I give you my apologies if I have made an error in my assumptions.
Unless things have changed, I think that for legal reasons you have to have a civil service as well, when you get '
married in a RC church. So it is understandable that you would obviously have to have a civil ceremony at some point too.
I would like to suggest one more thing, I think that if you and your DF try hard enough you will be able to get his mother to reconsider her stance (as I am sure she is desperate to do so) by making her believe how neither of you had thought it through, and that when you both did, your DF realised he would be devastated not to have his DMum there when he gets legally married, and that you also wanted her to be there when you realised that the legal ceremony would mean much more to her than to your own mother, because of the reasons I mentioned above. Good Luck OP. I hope that both of your Wedding Days are lovely 🩷