Thank you for your letter - and you're right: The time to deal with a broken relationship is now, not when it's too late. Not every broken relationship can be healed - but it's far better to attempt it than to be filled with guilt and regret because the opportunity has passed.
Why don't we try to heal a relationship that's been broken? Why do we allow the break to go on and on, without any effort on our part to change it? Sometimes it's because of pride - we don't want to admit we might have been at fault, and we don't want to have to apologize. Or sometimes, I suspect, it's because of fear - fear we'll be rejected, or we fear the other person will simply get angry at us and dredge up the problems of the past. You can probably think of other reasons why it's hard for us to reach out to someone we have hurt - or who has hurt us.
But these shouldn't keep us from trying to bridge the gap. The Bible says, "God has called us to live in peace" (1 Corinthians 7:15). It also says, "Make every effort to live in peace with all men" (Hebrews 12:14). If you need to reach out to others (perhaps members of your sister's family), do so today.
After all, this is what God did for us. We were separated from God because of sin - but Christ came to forgive us and bring us back to God by His death on the cross. Is your trust in Him?