Lifetime of being severely underweight, wondering what tests we can ask for or ideas of what might be wrong (21M, 24M)

Hi, I am making this post out of concern for myself (21M) and my brother (24M) being severely underweight our whole lives. Despite wanting to gain weight, due to the symptoms we experience, neither of us has been able to do so. When we try to increase food intake, we both tend to get strong physical reactions like fatigue, digestive discomfort, and feeling worse overall, which makes it hard to maintain eating more long-term. I am coming here to ask for help and see if anyone might know if there’s something specific that’s wrong with us. If this post isn’t allowed I’m sorry and I’ll take it down or post elsewhere. I’m not a doctor so I might say something wrong.

I am 21, 5’3, and currently 71 lbs. I have never weighed more than 80. My brother, who is 24, is somewhere between 5’4 and 5’6, and is currently 86lbs. He has never weighed more than 90lbs.

The reason I am including both of us is because we have the exact same problem, it doesn’t run in our family at all, everyone else is a healthy weight. I am scared that we are going to have severe health issues or die. I feel if I posted this about just one of us it might be written off as mental health issues. However, we do both have mental health issues because we had a very bad childhood. We thought it would improve after moving out but it’s been 4 years and hasn’t gotten easier. I am trying to understand if there is anything out there that would help explain this beyond just mental problems from childhood. We will be doing our best to get to a primary again, it’s hard because we aren’t well off, so a point in the right direction would really help.

My history:
Very little appetite
Childhood seizures (grew out of)
Many long-term digestive issues since childhood (IBS/constipation, chronic GERD, stomach pain, nausea, shaking, cold sweats)
Can’t tolerate lactose or gluten (even with lactase enzymes)
Frequent ER/urgent care visits for dehydration, heart palpitations, and adverse reactions to psychiatric medications
Chronic fatigue and difficulty focusing
Joint and/or muscle pain (unclear which)
Often feel worse after eating normal amounts or after exercise
When eating, I sometimes get a surge of adrenaline that makes me very nauseas and causes me to spit the food out and shake
History of potassium deficiency in past
Flexible since childhood / double-jointed fingers
Left rib that slips in and out of place, noticeable with movement (ex. mopping) and turning over/sleeping (usually not painful)
Weird ribcage shape on one side
Hospitalized twice as a minor for low weight (~1 month each time), but weight always returned due to ongoing difficulty eating and persistent digestive discomfort that didn’t improve
Eating often makes me feel extremely exhausted and “drained,” which makes it hard to maintain intake, it’s like the blood leaves the rest of my body
Muscle breakdown after a surgery (non-health related) that caused my lactic acid levels to go to 4.44 (the wake-up call that made me make this post)
Extensive therapy; therapist believes my issues are not fully explained by mental health

Mental health:
PTSD, ADHD, anxiety, depression, agoraphobia, depersonalization/derealization

My brother’s history (that I’m aware of):
Very little appetite
Severe difficulty focusing and completing tasks (loses track while doing things like making meals)
Episodes of confusion / “lapses,” now scared to drive long distances
Vitamin D deficiency (tested)
Fatigue
Kidney stone
Asthma (uses inhaler; not sure how formally diagnosed)
Unlike me, he feels better after walking/exercise
Flexible since childhood (not double-jointed)
Severe overbite and jaw issues causing chewing difficulty and jaw pain
Throat dryness and constant throat tightness even when hydrated with water/electrolytes
Trouble projecting voice

Mental health:
PTSD, maladaptive daydreaming, depersonalization/derealization

Family history:
Mother: cancer history, healthy weight, 5’4”
Father: skin issues history, healthy weight, 6’1”
Maternal grandmother: lupus history, healthy weight

I know nobody online can diagnose us, but does any of this sound familiar or point toward anything we should specifically look into or ask doctors about?