This screams scam to me. Venmo is not used for something like hospital bills; crowdfunding sites are. Also it's telling that it's his personal one.
While I can totally believe poor Brianna has a brain tumor, and I hope she does get the aid of doctors in getting it removed and keeping full quality of life, I don't trust Jack in the fucking slightest. That fat fucker is a pathological liar who is using an app well known for being mainly used by scammers or for small purchases. I would not be shocked in the slightest if a guy who tries to regularly lie about being sponsored would try to make money off his daughter in law having medical problems.
ESPECIALLY since money for him is money for food, meaning he can do things mommy-wife restricts him on barring tax breaks.