I've wanted to start saying I love you to my siblings, but can't bring myself to do it.
That's strange. Why don't you think you can express love for your siblings?
It has always been hard for me to be honest about my feelings.
I understand that it can be hard to show your feelings, but holding back your love is detrimental. Suppressing your feelings will lead to physical illness from blocked up energy.
I know. I wish it were easier to say the sorts of things.
It can be hard to talk about your feelings with your siblings, but you owe it to them to tell them that you love them. When you don't express yourself clearly to others, they may have doubts about your feelings for them.