Yes. To the point.

I guess I have an explanation:

onların ev-i : their home

This -i is possesive suffix for third person singular (o).

But as you can see we can use it for third person plural (onlar) too

So I think maybe we should think that -lar is plural suffix and -ı is possive suffix (for third person singular) when we use "ları" for more than one possession. That way maybe we can explain why we dont say "larlari".

Note: I am not sure