How to deal with Family members?

Merry Christmas and Happy Holidays to all!

Using FM8 Advanced, I'm doing pro-bono work setting up a new DB for a new church organization. 

We are tracking not only personal contact info but also personal interests, skills, classes taken, volunteer service positions held, financial contributions, etc. Some of these areas are easily identified as belonging to a particular individual, but other areas seem to overlap when there are other family members participating in their own right.

For example, some of this data, like home address and home phone are going to be the same as their spouse/partner — and will need to be automatically updated if one or the other changes. In other words, if a data entry person goes in to change the address field for John Smith's individual record, and if John is married to Sally and she has an individual record too, then I need to be sure that Sally's address field is also updated (automatically if possible, so as not to rely on the data entry person to know that the change affects someone else too). It's also the case that older children will begin to be tracked in the system, once they start volunteering, taking classes, etc.; so it's not just an issue of "couples."

And for financial contributions, if John and Sally are sending in one joint check, does it get recorded on John's record, or Sally's, or do I need to create a joint record??  This needs to be clear not only for data entry, but also for reporting.

And sometimes the church will want to send out form letters — sending something out to individuals, and sometimes to a couple or a family; so the issue comes up as to how to determine which names, salutations, etc. are used on label layouts and letters.

Hence my timely holiday subject: How to deal with family members?

Thanks in advance for any light you can shed on how best to go about this.
Victoria