Airport Extreme Base Station + Express Repeaters Question

I use an Airport Extreme as the base station and two Expresses are repeaters.

When viewed in Airport Utility, normally the signal comes out of the Extreme then branches out to each Express, which gives me the best coverage in our house and guesthouse (as shown).

Sometime, out in the guesthouse the signal degrades and checking Airport Utility I see that the signal goes in a straight line from the Extreme to the Express in the house and from that to the guesthouse Express.

Checking to see which devices are working off of each source, devices in the guesthouse are connecting to the base station and nothing's connected to the guesthouse express (in the branch mode, devices connect to the wifi source they're closest to).

My question is this: is there a way to "lock" the guesthouse Express to the Extreme base station?

Thanks for the advice!