BIL is now saying he lied about my FIL sexually abusing him. What do I believe

Sorry if this is complicated but my husband (31m). Has a younger brother (23m). My FIL is in prison on a 30 year sentence for sexually abusing my BIL from 2013 to 2018. My BIL is very heavily on drugs. It's bad. He's 23 and lost almost his entire top teeth. My husband tries to help my BIL as much as he can but he keeps making bad decisions after bad decision. Last week my BIL told my husband he's back in contact with their dad. His dad may get out in 15 years instead of 30. My husband was telling him he shouldn't be in contact after everything and his brother goes "Oh that didn't actually happen."

My husband isn't taking his brother saying that seriously. I feel like maybe it should be but here's where it gets complicated and I'm wondering should him saying it never happened be taken seriously. Here are some things that contradict his brother saying that never happened.

1. My FIL plead guilty. He admitted to these things and I'm pretty sure signed an affidavit saying he would molest and sodomize his own son.

2. He also admitted guilt over a phone call before he was arrested.

3. According to my husband. They found my FILs semen in places on my BIL it shouldn't be

4. There's STRONG evidence their mom knew which is why my husband doesn't talk to his mom anymore.

I'm just curious on why he would back track now and claim it never happened

My husband thinks is brain is just really messed up because not only is he on drugs. He's always had bad seizures and it's taking his medication. He gets bad memory gaps from his seizures. One bad one he has several months ago he lost a few years of memory for a short period and my MIL used that as a way to steal all his money. So I would you take him now denying that his dad did anything seriously? My husband isn't