Jenn, just a killjoy note that if your step-mother is taking an autosomal DNA test--the most common kind, like the ones you can buy from AncestryDNA or 23andMe--it won't be of any use as genealogical evidence back to Sir Robert Parke or Martha Chaplin. Just far, far too long ago. Robert's Y-chromosome data and Martha's mitochondrial DNA (mtDNA) would still be intact and verifiable, but not the autosomal DNA...the part that makes up 98.2% of our genome.
The reason is that our DNA first goes through a mix-master during meiosis called crossover, and then it's commingled with the 50% our spouse contributes. Nature's way of assuring enough genetic diversity. Since (almost certainly) we aren't testing against ancient remains, we have to look for matches that are in a generation contemporary with our own. And since the first autosomal direct-to-consumer test wasn't offered until 2007, it means we have to match with someone who was alive to take a test in the past decade..
What becomes important in the ability to use autosomal DNA as evidence is the number of birth events--the number of times DNA is scrambled and commingled--in the family branches between the two test takers. At the distance of 6th cousins, the expected amount of shared DNA becomes functionally zero (0.01% in fact). That equates to 14 birth events, with 5th-great-grandparents the most recent common ancestors.
In truth, the amount of detectable DNA is likely to zero-out before that. One important study on the subject showed that only 4.1% of your actual 6th cousins will share any detectable autosomal DNA with you; for 5th cousins, it's 14.9%; at the level of 4th cousins you're still batting a little under .500.
Autosomal DNA is really, really useful at 3rd cousin relationships and closer. It takes some knowledge, effort, and time, but 4th and even 5th cousins can provide genealogical evidence. Beyond that, though, autosomal DNA isn't of much use if accuracy is a priority.
Putting it in terms of number of generations from the test taker, with that person counting as generation zero: you're golden to three generations back; silver to four generations; bronze to five generations; at six generations you might be still in the race, but you're a long-shot; and at seven generations and beyond, you probably didn't qualify in a preliminary race and never made it to the final. If you extend that to years by assuming the test taker is, say, 50, and each previous generation was a generous 30 years, it's unlikely autosomal DNA will be of any genealogical use beyond 200 years ago...and almost certainly will be of no use beyond 230-250 years ago.
Now that I'm finished with the sports metaphors...have fun with the search!