I think it's important to adjust your thinking to incorporate the fact that these two are now a separate entity. If you wouldn't behave a certain way with any other adult couple, tread lightly before behaving that way with them, whether together or separately.
I would not speak to future DIL about it but would always keep to direct lines of speaking to your blood relative about any issue you have. In my experience, it keeps mess smaller. And when I haven't done so, I've usually been sorry.
And remember that manners still apply, even with family, namely the one about not inviting yourself over to other people's homes. You say you've "repeatedly" tried to do so and that you are "getting tired of being turned down." I understand that you want to see where your son lives and that you mean well, but that doesn't make it your right. You say you "don't want to intrude," yet what else would you call repeatedly asking after you've been told "no?" Imagine how they feel to have to continually come up with excuses because you keep pushing?
Also, keep in mind that your future DIL's feelings about her mother's dementia are not about how you feel about it; they're about how she feels about it. So I don't think explaining to her that you really don't mind it is getting the point.
And, this latest reason could also be another excuse and not the real reason. Perhaps the real reason is more personal and they don't want to share that information with you right now- which is their right, even though of course you would prefer to be included.
For example, perhaps she/they are just overly stressed right now and feel they'd have to put in an exhausting amount of effort to make the house what they'd consider presentable to you. Or perhaps part of the MIL's dementia is such that she feels you are a threat. Or many other things. Their not wanting you to come over (while still coming to visit you at your home) can actually signify that the future DIL (and maybe your son too) cares very much about making a good impression on you. So let them. Attempting to get in there anyway even if it makes her (or them) feel exposed or whatever seems a bit tone deaf.
I'd be very careful not to get into a power struggle with future DIL but instead, accept that it's her/their right to set any limits she/they want in their lives and home. People who don't feel engaged in a power struggle tend to relax and let those walls down- when they feel ready and able to.
I think it will likely resolve itself in time and that the wise thing to do in the meantime is drop it completely. As in don't say one more word about it to either of them. Otherwise, you risk becoming associated in their minds, not as the great mom and MIL we all want to be, but as a Problem Person who needs to be Managed. I wouldn't like this boundary either but for a good relationship with them both, it is more important to show that you respect the boundaries they set and their right to not have to give explanations on demand, than it is to get to visit their home.