Sometimes the best way is to lead by example - it's hard for someone to get onboard with saving for investment properties if you've only got $700 in a savings account, and you're trying to sell the "no down payment!" method - you know what I mean?
Also - consider the idea that they have NO IDEA what you're talking about when it comes to rentals and the potential of them, I knew nothing about RE a year ago. I got more into it because of my own interest, but as you know, some people just aren't interested in it, period - that's okay, but it shouldn't allow them to be able to affect your future.
It took several months, I didn't really notice how long, until it began sinking in to my gf that the whole "you only live once" and "you have to enjoy today" was complete bullsh*t and that you need to plan for your future or you won't have one. Eventually, after being subjected to months of me talking about real estate/money (I had no property yet), listening to the podcasts in the car, looking for properties, eventually buying one, etc - she doesn't mind the idea and has actually saved 20k in the last 1.5yrs or so (not necessarily for a RE purchase, but just because I talk about money ALL OF THE TIME, I think she finally realized that her bills are X and her income is Y... so she should be able to save Z per month - she just wasn't focused on that before - and as you know, you're money disappears unless you tell it what to do).
People who have zero financial literacy (which was me until very recently) have a hard time grasping frugality and let alone real estate investing - so, like I said, one way to get them on board is sheerly to walk the walk and show her how it works, save the money, listen to the podcasts, gain the knowledge, eventually buy the rental, etc etc.
I don't know the whole situation of your entire financial picture, but it seems like there needs to be a serious talk about your future financial goals - what are the goals for retirement? Are you saving enough for that? What's your monthly budget look like? Her lack of interest in future financial goals cannot go ignored, but maybe it's as easy as her gaining more knowledge of money, RE, and finances and the "FIRE" community before it clicks.
Edit: ALSO - totally agree with the 3rd party thing! Go to an RE meet up, so they can see that OTHER PEOPLE DO THIS, IT WORKS! They live in your town, they like to talk about what they do. I think it makes you feel less scared when you realize other people already do it, and you can too.