Parents want to give a gift - give it without ties - why do they feel the need to let everybody know - to bask in glory?
I think it's probably okay to want the person GETTING the money to know about it though. As they are married, this money is a marital asset, not the brother's alone.
I agree with the posters above - I think that the brother is planning to leave his wife and has some wrongheaded notion that if she doesn't know the money exists (even though she will know perfectly well) she won't be entitled to any of it, even the return of the money she has bailed him out with in the past.
Even if he is NOT planning to leave her, if my husband was given a substantial sum of money by his parents and actively concealed it from me, I would consider our relationship to be over. Not because I wanted "his" money, but because it would indicate that the trust had died and that he did not consider me to be his partner any more.
@1strangerthings - your sister in law will know perfectly well that there is money. She will know that your parents are selling the house, and she will observe that you and your wife suddenly paid off your mortgage, or got new cars or something in the immediate months after. She will have access to her husband's finances (presumably?) and she will see that he has been given a lump sum and concealed it from her. She will also know that you colluded in this.
It's not your place to literally ring her up and bare your soul to her, but you should advise your parents not to be complicit in lying to her about this.
Suggest that they sit your brother and his wife down together - point out that as your brother is demonstrably shit with money, and has had to be rescued by his wife in the past, they risk him losing this money almost immediately, rather than it benefitting his children and overall lifestyle, as they hope.
Impress upon them that as your SIL is such a competent person, the money will be put to much better long term use in their joint hands than in his alone - if he is lying about it before he even gets it, he is likely to be intending something half-witted for it - paying off secret debts springs to mind, gambling, a bad investment, or wasting it on material crap.