She knew that the only thing she could rely on was his love. If he loved her, she could have everything. If he didn't love her, she would have nothing.
So, she needed to know just how much he loved her and what he could do for her.
She had an account in her heart. The reason for hiding the matter about Thomas was partly because she was worried about the little guy's heart condition and could not be stimulated. On the other hand, she wanted to test Robert's intentions. If he could withstand the pressure and still let her marry into the Thomson Family, even when he knew she had an "illegitimate child", it would be a way to clear her name from the grievance of five months ago. At the same time, she would be able to put aside all her worries and commit wholeheartedly to him!
Call her shrewd or ruthless if you want, but she was such a realistic woman. The only thing she could rely on was Robert's love, so of course she needed to know just how important that love was on his scales!
