I have a deceased family member who I'm interested in working

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I have a deceased family member who I'm interested in working with, he's made his presence known to me in the past and has helped me through some really bad emergencies.
I believe he may be at unrest because his ashes were never scattered (I'm working on that), but his mother was hurt terribly by his death and still isn't ready to part with them.
If I try to invoke him, will that hurt him? I've been told invoking spirits of the dead is technically necromancy and that necromancy hurts the eternal soul.
How can I go about asking if we can keep his ashes? Would keeping a small amount for an idol be alright, or will he most likely still be stuck in between worlds?
Any advice for general ancestral magic?