It's easy for people to judge sins of comission harder because they are the "visible" sins. Sometimes a sin of ommission can go unnoticed for days, months or even years and therefore seem less serious. God is clear in the Bible though that He sees all things and that there is no partiality with Him. A sin is sin whether it is high profile or private.
It's easy to justify sins of omission as well. After all, we're not actually murdering someone, committing immorality or getting drunk right? What's so bad about walking by on the other side? Wrong!
Failing to love is a violation of the 2nd greatest commandment! God said to love Him and to love others. Failure to care, failure to get involved in other's lives when they need our help, failure to think of others instead of just ourselves is failure to obey.
Our problem is that we do not think God's thoughts. We think very superficially as people. If it looks bad on the outside it's bad we say. We tend to sweep the other stuff under the rug. Our second problem is that we are selfish. We tend to center our standards and convictions around ourselves. If it is convenient for me to serve God in this way or that, it's right. If it's not convenient for me to do this or that it's not that bad. This is self-justification.
On an encouraging note, God will provide all that we need. He may not provide all that you want. In the example of your mother, God may not have provided enough money to give high end drapes, but He provided enough money to care for her family...however, she chose to do something she wanted - decorate her house. When we struggle to get involved in other people's lives because of our selfishness, we are driven to the cross - we are often called to sacrifice personally in order to humbly follow Christ's call to love Him and others. Jesus was the ultimate example of sacrificing comfort, convenience, companionships, ect in order to help a world of people in need. God will give the strength and wisdom to learn to be like Him. Being willing to care for people despite personal inconvenience reaps dividends we cannot even measure. And these dividends aren't just in the lives of others we care for, they are in our own hearts as well.
The happiest people don't have all that they want; the happiest people want all that they have! My encouragement to you is to not allow the failure and sins of commission of those in your life to keep you from following Christ's call to love Him and others. Don't let the loss you experienced hold you back from letting God's love flow through your life to show care and involvement to others lest you fail someone as you were failed. Overcome evil with good! If you need resources to help you forgive / overcome evil with good, check out this website I have personally found so so helpful: http://r12online.livingontheedge.org/r12online/supernatural/
So, is failure to love, failure to care, failure to be involved... is this selfishness "worse" or "less worse" than another sin? Well, we know this, the Bible clearly says how can you say you love God if you do not love your brother? And to not love God is the highest offense we can make. We need God's mercy and grace to bring us to our knees for help and hope to be like Christ.
God bless you.