Agreed. I may make the occasional crack about how DS's SFX look now (particularly what I like to call simply "Angelique's Floatin' Head"), but I can appreciate it as being revolutionary for TV in 1967.
Though it is in my profile, I've been wary of discussing my age here before, as I thought it might attach a negative stigma to me (I'm a long-time member of a forum that has many very ageist members, and I rarely visit their public boards now because of said ageism), but I feel a bit more comfortable discussing it now that the (fantastic) community here has seemingly accepted me so much, so here we go.
I'm 14. That's not a typo, and I'm not kidding; I was born 28 years after the original and 3 after the revival. My mother was a big fan when the show first aired, and the reruns on the Sci-Fi Channel were morning staples in the house, though I was too young to fully appreciate them. She'd told me a bit about the show and I decided I wanted to get in on it, so I offered the suggestion of signing up for Netflix to get the series, which we did, along with buying a few collections.
Where my age comes into play here is the fact that my friends are around the same age as me, so the majority of them have never even heard of Dark Shadows. Most of them have proven themselves mature enough to accept older movies and TV shows, but I've had horrible luck convincing them to watch Buffy (not that it's an "older" TV show by any stretch of the imagination), which is only 144 episodes long, as opposed to DS's 1,225, so I haven't even bothered to attempt luring them into DS. The only thing I've ever been successfully able to bait someone into was the anime series Death Note, which is only 37 episodes long, plus I had the help of a friend who was also a fan (who, coincidentally, hooked me on it, despite the fact that I generally dislike anime).
In addition to this, my already scornful grandparents seem to think even less of me than they did before ever since I started watching DS, even flat-out telling me that I shouldn't watch the show. They have a big thing against fiction dealing with the supernatural in a non-religious context, and have simply refused to watch even the first storylines with no supernatural elements. But I don't really care what they think. I enjoy the show, and if they have a problem with it, well, that's just too bad.
So I basically live in a situation where convincing anyone to watch DS is impossible. In the case of your brother and his partner, I'd say that it's probably impossible to get them into it. Some people just can't be convinced to get into something, and when that something has over a thousand episodes, the likelihood of them being convinced is even lower. My best (and only) advice is to make sure it's always playing on your TV whenever you have either of them over. With any luck, they'll get hooked on something they see in that time.