Only two brother-sister combinations have ever won on the elite tours and none have managed the feat in the same week.
The Thompsons are right in the hunt to become the third duo, however, after Bill Kratzert and his sister Cathy Gerring, and Jackie Gallagher-Smith and brother Jim Gallagher.
Thompson opened with a bogey but reeled off eight birdies in his last 15 holes to move to 17-under for the tournament. Summerhays shot a solid 69 to join him at the top.
Americans crammed the top nine spots of the leaderboard with Chad Campbell (65), Cameron Beckman (65), Woody Austin (67) and Kyle Reifers (67) sharing third on 15-under.
Brendon Todd (66), Vaughn Taylor (68) and Bill Lunde (68) were a stroke further back at 14-under.
Thompson, who was on the main tour previously in 2006 and from 2008-2010, returned this year as a graduate from the 2012 web.com Tour but has missed seven of his last 12 cuts and posted one top-10 finish back in January.
"I've been playing good (this week). I got off to a little bit of a rough start with a three"‘putt but after that it was pretty good," Thompson told reporters.
"You know what, there is always a possibility," he added of the potential family double.
"I just have to go out there and play like I have the last couple of days.
"I don't know what score it will take, but it's going to be a good one. The guys have gone low and the golf course is in perfect condition."
(Reporting by Ben Everill; Editing by Ian Ransom)
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