the node, channels, liquidity is same for everybody.
Does each of your family members has separate channels connected to different lightning nodes?
AFAIK, each custodial account should have a "virtual balance" that's managed by your lightning node rather than actual channel capacity.
Unfortunately, now that it's clear, I came up with the same question why it was routed.
Is this consistent to other transactions between your custodial accounts?
If not, investigate the wallets where it occurred, both sender and receiver.