absive x brother

so my x is absive i left him when our son was young he has an older brother i also cut out the family ro after giving them three chances to keep his father an my son relationship separate from theres with my son due to him being an addict an abusive but they disnt listen to me an they use to go bwhind my back an put my son at risk with letting him bw around his bio father. not okay talked to them about it plenty of times even took my son away couple month an gave them more then one chance. one day i just left said was visiting my mom in a differnt state when i wasnt changed my number an havent been in contact for over two years. my son is now 4. his brother that i forgot about an forgot to block messaged me on fb messager saying he has desticular cancer an an going through chemo an wanted to see my son his nephew. my son doesn't know him or his family nor his bio dad he calls my boyfriend who been raising him for two year dad doesnt know anything eles also pregnant with his daughter i never wrote him back an i think i choose to leave it alone an let it be u think thats wrong of me?? i know if I respond that it will be turning my son's life upside down also would be setting myself up