実家のPCは僕が遠隔操作する時のために全部ローカルIPを固定にしてるのですが、妹傘のスマホはそうではないので、
母傘のPCがついてない時に「ここ空いてるー！」とスマホがIPを握ってしまって母傘のPCがネットに繋がらなくなるということですね。
妹傘のスマホもIP固定にする必要があるのですが、サブネットマスクやらDNS設定やらの説明は口ではムリだよなーと(^^;)

It's because I fixed all the PCs in my folks' home to a local IP so I could operate them remotely, but that didn't hold for Kid Sis-gasa's smartphone. When Mama-gasa's PC wasn't on, the smartphone would think, "This slot's available!" and snag the IP address, so Mama-gasa's PC would stop connecting to the 'net. I'd have to set Kid Sis-gasa's smartphone to a fixed IP as well, but I figured that verbally explaining stuff like subnet masks and DNS settings would be asking too much. (^^;)