Need help understanding a situation

Hey all, so I need help understanding a situation between my FH and FSIL. They got into a huge argument about 3 weeks ago. Some things were said that aren't good; such as my FSIL accusing her brother (my FH) of not caring about her family and his nieces. She wanted us to take her daughters for 3 weeks; my FH just recently found a new job because his company went under due to the virus. They haven't given him a for sure start date but said it would be in May; he didn't want to say yes when we didn't know when he would be starting. There were some other things said as well such as my FH and I are selfish. I'm an only child, so I'm wondering if this is normal for a brother and sister to fight like this. I'm not use to this dynamic. To me it seems they are being very hurtful to one another; my personal opinion is that my FSIL fights a lot with her husband and ends up taking it out on her family members. She hasn't spoken to my FH or her dad since the fight.