This piece of information came from Eric Lindstrom who worked at Crystal back in the LAU days. He was fired after Underworld underperformed and subsequently answered some fan questions over at TRF. Toby was merely working as a consultant on the games so he wasn't able to make any decisions but, according to Eric, he was the one who made the initial suggestion to have Lara look for her lost mother and, to me, him even thinking that that could be a good idea tells me that he doesn't understand Lara as well as someone who had such an influence on her characterisation/presentation should have. That being said, a lot of the other things Lindstrom said about how he and Toby viewed Lara (a "well-behaved psychopath") sound spot-on which makes it all-the-more surprising and disappointing that they thought the whole parents BS was a good idea.
Here's a link to a Wiki article detailing Eric's TRF posts (they say that the user wasn't sufficiently verified but TRF's admin is very trustworthy - if he said it was Eric, then it was Eric):
https://www.wikiraider.com/index.php/The_“Eric_Lindström”_posts