You can't make them come, they come if they want to. It's not like you are taking them away from tea time.
In all of my experiences, if they come, they want to help.
That I very much agree with. When my ancestors show up its because they want and desire to do so. Sometimes I think its for my best interest, other times I think it's for their own best interest or desire. Ironically I suppose but most of my ancestral dealings arise more from a genealogical aspect than from a witchy aspect of things. Though there have been a couple of times where it was to use me as a talking stick or hollow bone to convey a message or give guidance within the family when they could not reach the person.
The RIP aspect to me is an important one, especially as I relate a lot to the Greek notion of restless dead and the ability to summon or command those dead who did not get proper burial and ritual when they dropped their earthly garb. In that capacity to Rest In Peace to me is to be in a state where their spirit is at peace and they are not subject to a condition where they could be called and commanded to perform some action with the promise of things being done to punish or reward them so they can cross. Nor do they lay in a state where some failure on their own part keeps their spirit attached to this world. Such as my ancestor whose spirit failed to protect and was forced to haunt until a blood relative returned to Scotland so he could protect and fulfill his promise. An honor promise that binds even after death when it results in loss of honor and word.