Godishelpingme - not sure from your post just what you are trying to help your friend with, so it's hard to offer up any specific advice.
So which is worse???
Mom....3 long-term affairs with an abortion (which she believes and was raised to believe is morally wrong...)
Dad....ONS with mom's sister
If what you are trying to do is to "rank" the offensiveness of sins, you can do that. But the "seriousness" of one sin versus another sin is not just based on the "worse-ness" of one sin (say, lying versus adultery, where one is obviously much more harmful than the other). It is also based upon the perception of the "offended" person as to what "bothers them" more.
At the end, it has more to do, imho, with a person's own "worldview," their beliefs, etc.
To a lot of people "sin" isn't sin because they don't want it to be, so the question really does impact on what someone's "Standards" are and why they choose a particular of set of Standards for themselves.
It is unclear WHAT you are trying to help your friend with, but it obvious from the little you posted that your friend may well be "struggling" with the reality that her parents are "human" and that their behaviors might not be a "role model to emulate" for herself.
It's also unclear if you are trying to help your "friend" or your sister (possibly the same person?).
Since MY "worldview" involves God, as yours may, then the "way" to help would seem to be talking about God and the Standards that HE has given us that are applicable to everyone simply because He IS the "Standard" and He has the "authority" to determine what IS and what is NOT "sin." All sin is "anathema" to God (intolerable and incapable of 'living with' may give an approximation of the seriousness of any sin to God), and it would seem that this situation might be a timely way to talk about the REALITY of sin, regardless of anyone's "excuses" for why they CHOSE to sin.
The reason is simple. Sin is always first and foremost against God. What your question implies is that because we humans may consider one "offense" to be "worse than" another sin it is "easier" to "forgive" one sin versus another. The reality is that ALL sin is so bad that it COST God the life of Jesus Christ in order for GOD to be able to offer forgiveness. That "debt" has been paid in full, but ONLY for those who surrender their lives TO Christ. Other than that way, the rest is just "human rationalization" of their ability to "play god" as if THEY were the "ultimate authority" on what is "right and wrong" and what they will choose to "forgive or not forgive."
The other reality of sin is that our actions, good or bad, always have consequences. Certainly the "consequences" of some offenses (sins) are less than others as to how they may or may not "impact" our relationships, but there is NO escaping the fact that Adultery of any kind severely impacts both the relationship between husband and wife AND the relationship of the offenders with others (especially with children who, by nature, look to their parents as "Role Models" of "how to behave and how to make 'good choices' in one's life).
It may sound trite, will sound trite to those who do not know Christ, but the real offer of help and hope is contained in just two words....Love God.
If you want it "expanded" a little, then look at Mark 12:30-31 (known as the Greatest and Second Greatest Commandments) OF God to all humans. Love "as Christ first loved us."
If you want to discuss the "serious" of one sin versus another, then I would direct you to John 8:7-11. We are ALL sinners and fall short of God's Standards. "Then neither do I condemn you," Jesus said, "Go now and leave your life of sin."
The question is very simple; where someone's "heart" is, is what is important. No one can serve both God and their own selfish desires, because when they do, they WILL sin. "Getting right with God" will also result in "gettin right with one's self and their CHOICES in life." That will determine whether they bring honor or dishonor to God "from this day forward," recognizing that ONLY God can forgive sin and establish a "right relationship." Without God, man (even those who don't recognize that God exists) will forever KNOW that they have done wrong and will try to "hide" it just as Adam and Eve tried to "hide" from God. God knew their distress since they knew that they had, in fact, put themselves ahead of God, and He gave them "coverings" to help "hide" their nakedness. Consequences of sin remain, and God has provided all that is needed to "overcome" sin, but WE have to accept what He has made available. Once again, we have to CHOOSE.
I don't know if any of that may be of help to you, but there it is. The "sins" can be "ranked," and they can even be forgiven. But the consequences of the sins may continue to impact the relationships, most likely WILL continue to negatively impact the relationships if God is not a "part" of the recovery and restoration process.
Always we can "do it" with or without God, with God's Standards or with those of our own "choosing." We WILL "reap what we sow" because only God has the power to transform hearts. And only God has the power to IMPLEMENT Romans 8:28 and USE the "bad things" to "grow" a newer and better crop instead of "more of the same old bad seed" crop.
God bless.
