Sounds OK to me!

His father was an opium-addict and three, out of nine children of my grandmother, died of easily curable diseases due to poor sanitation and lack of sources.

its too long…

His father was addicted to opium. Three of the nine children my grandmother had died from easily curable diseases due to poor sanitation and lack of available resources.

hah idk it still sounds long

I think, the ideas in your sentence are connected in a way but it’s not good to have them mixed up cuz the sentence sounds incoherent. You may be trying to describe how miserable the family is, but you fail to send the message across. You can disect the parts of the sentence if you opt to. It could be that you create two or more sentences out of that long sentence. try also to clear out the objects of your pronoun. Like you said, “his father”, then you were suddenly referring to your grandmother on the next… You could try to fix it up by creating two sentences or you could disregard any unnecessary information in the sentence rather. in that way your sentnece would be better understood.

Instead of “and of the nine children my grandmother had…” you might consider replacing it was “and of his eight siblings…”. It just sounded somewhat ambiguous, that’s all.

its ok with me