Reneging

Starting 1 week from now and signed the offer 2 weeks ago, but got an offer elsewhere (unexpected) for 100k more, just today. Is there any way to renege on this without burning a bridge? I was excited to join, but I want to take the higher comp opportunity. Both are similar scope/role. Do I share the reason (better offer?) I do have a family situation (brother with cancer) that I could use (he even said just say that and hope they'll feel bad and not blacklist you), but perhaps they'd be willing to wait 4-5 months for me to join, which would be bad. What's the best play here to reduce chances of blacklisting?