I did some thinking about why Okiya (assuming he is Akai) would suddenly step up his protection of Ai after Sera appeared assuming Sera is his sister and also not a bad guy or part of the Black Org.
Here are events from Okiya’s perspective:
We know he's been tapping the Agasa house, at least since recently. Discussions about Sera that took place in Agasa’s house in the same vicinity as the overheard conversation include.
771 pg 10: Conan talks about how Sera seemed to know about him ahead of time. Haibara said “Then, that detective named Sera knew both your name and that you were a detective?” Conan confirms this but points out Sera seemed to throw the case on purpose for some reason. Haibara then began yelling that it was bad to use Kudo’s voice in front of someone suspicious like Sera. Conan then says Sera feels familiar and doesn’t feel like an enemy.
772 pg 4: Conan mentioned that Sera was involved with the current case and is over at the Mouri detective agency.
Sera may be using an alias because her family name isn’t Akai. Okiya may not recognize it via listening device. (Though it could be possible Akai is using the alias or "Sera" has become the Akai family witness protection name) I also assume Akai would recognize Sera in person -- that is she isn’t using a disguise or drastically changed her appearance.
Later, Okiya saw Sera outside of Agasa’s house talking with Mitsuhiko and Genta while looking up at it. I assume Okiya had been watching her. I also assume Okiya can’t hear what they are talking about.
So why the extra protection?
Five possibilities occurred to me:
#1. Okiya can't be sure Conan and Haibara's Sera person that he overheard the discussion about via listening devices was in fact the person talking to Mitsuhiko and Genta. If Sera has been using an alias, this makes sense.
#2. Okiya couldn't be absolutely sure if that person was his sister and not someone in disguise,
#3. and even if that person was his sister, he couldn't be sure that she wasn't being watched or,
#4. unaware of the danger of the Org, he is worried she might accidentally do something that would endanger Haibara and Conan's covers.
#5. He thinks his sister may attempt a direct confrontation with either Haibara or Conan. He’s trying to head it off.
#1: Okiya would want to step in because this means there could be someone threatening Conan's identity.
#2: Okiya has already seen someone disguising as Akai wandering around town, so keeping that in mind, perhaps if someone in the Black Org has done some digging and found out about Akai's family, that person could be using their appearance to try to lure him out and gather information. I am specifically thinking Okiya suspects Vermouth, who knows about Conan and Haibara and might think they helped Akai fake his death and is trying to dig for info. I assume Okiya can't hear the conversation through the window, so he doesn't know what Sera is up to, only that she appeared in front of the Kudo residence all of a sudden and is talking to the DBs.
#3 & #4: Sera probably came over on her own accord, and not knowing the danger, may not have taken proper precautions against making sure the BO don't find out who she is. Okiya is worried that something may have already happened.
#5: This would imply Sera is very certain of her deduction or else her methods aren’t very subtle. I’m not sure if she thinks this strategy would work although she did want to meet Conan at the beginning of the Mouri agency hostage case. Also if he thinks Sera is going to do such a thing, he must also realize that the way he is following the DB isn’t exactly subtle and may put him on her radar.
I still feel somewhat unsatisfied with this. I think more info on Sera or what Okiya knows is needed.