How to introduce some properties and logic into my many-to-many relation

Description:
I have a main FamilyMember class. I want to figure out how to define a marriage between two different FamilyMember objects. I also have some more information that I would like to store about a marriage (date, place, etc.). Also the system needs to be able to support multiple marriages for any FamilyMember object. Ideally I would like the system to restrict a marriage relationship to only two FamilyMember objects. How can I define a custom many to many relationship between two different objects in the same class? Also, how can I define these associations?

Answer:
We would advise you to create a many-to-many association between the FamilyMember and Marriage classes and introduce a new class which will control marriages:

C#
public class FamilyMember : XPObject {  
    protected override void BeforeSave() {  
        base.BeforeSave();  
        MarriagesController.CheckFamilyMember(this);  
    }  
    public string FirstName;  
    public string LastName;  
    [Association("FamilyMember-Marriages", typeof(Marriage))]  
    public XPCollection Marriages { get { return GetCollection("Marriages"); } }  
}  

public class Marriage : XPObject {  
    protected override void BeforeSave() {  
        base.BeforeSave();  
        MarriagesController.CheckMarriage(this);  
    }  
    public string Place;  
    public DateTime Date;  
    [Association("FamilyMember-Marriages", typeof(FamilyMember))]  
    public XPCollection Partners { get { return GetCollection("Partners"); } }  
}  

public class MarriagesController {  
    public static MarriagesController CurrentController = new  
MarriagesController();  
    public static void CheckFamilyMember(FamilyMember familyMember) {  
        CurrentController.Check(familyMember);  
    }  
    public static void CheckMarriage(Marriage marriage) {  
        foreach(FamilyMember partner in marriage.Partners) {  
            CheckFamilyMember(partner);  
        }  
    }  
    public virtual void Check(FamilyMember familyMember) {}  
}  

Now you can control the marriage rules via the CurrentController property.
For instance, you can check the number of people participating and count the number of marriages:

C#
public class MyMarriagesController : MarriagesController {  
    public override void Check(FamilyMember familyMember) {  
        if (familyMember.Marriages.Count > 1) {  
            throw new Exception("...");  
        }  
        foreach(Marriage marriage in familyMember.Marriages) {  
            if (marriage.Partners.Count != 2) {  
                throw new Exception("...");  
            }  
        }  
    }  
}  

Another way is to create your own persistent object for this many-to-many relation and implement the necessary rules within its BeforeSave method:

C#
public class FamilyMemberToMarriageLink : XPObject {  
    protected override void BeforeSave() {  
        base.BeforeSave();  
        //...  
    }  
    public FamilyMember FamilyMember;  
    public Marriage Marriage;  
}  

See also:
How to change the name of the underlying table and columns for a many-to-many relationship
What is the best solution for mapping a many-to-many relation to an existing database?
How to delete an object which is participating in a many-to-many relation