Except the government will just deny what Laura K has said and accuse the BBC of spreading fake News. That is why you need official on the record statements and not tittle-tattle.
Sadly it's not an either/or. Last night the alternative to her reporting the 'unnamed source' was probably radio silence rather than an on the record statement.
If Laura K (or someone else) said no, I won't report that unnamed then we would not have heard the excuse last night (and therefore the inconsistency found this morning wouldn’t have been an inconsistency). All we’d know is what’s come from the official lines this morning.
Notice how this morning’s official lines from no 10 now have to directly discuss his sister? They probably wouldn’t have without the excuse last night and the advance this morning (when people said hmm that’s a bad excuse, don’t you have friends/family locally?)
And if it turns out those lines on his sister don’t tally/line up, then the government will face even more pressure