Making Family Understand your Condition

Hello, I am 26 years old with psoriatic arthritis. I was diagonosed at 18, I was in remission for years but about 3 years ago I had a bad flare up. My condition has gradually got worse with me having multiple joint steriod injections, steriods, upping my medication.

I am now due to start biological treatment in the coming weeks, I'm very nervous. However, my boyfriend and his family seem to not fully understand the extent of my condition. They have made comments about my "dodgey knees" as well as me feeling tired 24/7 and needing to nap.

I am at a dead end as how to make them understand how bad my condition really is. Does anyone have any tips on how to speak to them about it? Or make them understand how bad it really is?