HG,
Sometimes, life just gets to us.
It's ok to feel down once in a while, as long as we don't stay there. In fact, it's quite "normal" whatever that is, lol! But, as my friend, Ann, says, "The difference between a bad day and a good day is about three days." In other words, this too shall pass. It just sucks going through it!
If you continue to feel depressed after a few days or more, though, definately do consider talking to a counselor or your doctor. Ongoing stress can and does lead to depression. You've been through some stressors lately with the b/f's son and more.
And resentments over the sister aren't doing you any good either. Please try to let it go. Remember, a resentment is like drinking poison yourself, thinking it will kill the other person.
I've found, for myself, that the best antidote for resentment is prayer. Not for me, but, for the one whom I have a resentment against. They are the ones who are sick and need help. Remember what Jesus said as he died on the cross, "Forgive them Father, for the know not what they do." It was the Romans who were sick; who were causing his pain. He turned it over to G*D and was able to let it go. Only then did he die in peace. That, too, is what we must do. Pray for those who are our source of pain; give it to G*D; and let it go.
I hope this helps. Of course, continue to come here and talk it out. There are many who can share their own experience, strength and hope.
Shalom!