Congratulations on your new little boy! Caring for two little ones is an adjustment, but millions of woman have accomplished it, and so will you. I have confidence in you.
Expect things to be a little confused for a while. You may not be able to get into a real routine as you might have had before your son was born. But that's all right - you'll all survive. I'm so glad you've been able to have some help. Ask your husband (and your mother, if she lives nearby) to continue helping. Welcome any offer of help that is given! You ARE going to get used to taking care of your children, but you ARE going to be tired, so take advantage of any offers of help with the housework.
Your son is less than two weeks old. You can pad a laundry basket or even a big box if you don't have a bassinet or a portacrib. When he falls asleep with a full tummy, you can put him down right near you.
When you change his diaper, let your daughter watch. Ask her to hold the clean diaper for you and hand it to her when you need it. Explain what you are doing. Babies don't know as much about going to the bathroom as she does (I don't know if she's still in diapers or not, but you can adapt this to the situation), so his diaper has to be changed very often. He can't say, "Mama, I'm hungry, I'm tired, I've got a tummyache," so he has to cry instead, and we try to guess what he wants. Little Brother can't even hold his head up! - "Aren't you glad you can hold your head up?" - so we are very careful when we hold him. Keep explaining. When you're nursing, that's a great time to have Big Sister bring a book for you to read to her. You can count things in the room. You can try some ABCs of what's in the room. There are a lot of things you can do with Big Sister while you're attending to Little Brother's needs.
Do you have some music you both can listen to - both "kid stuff" and grown-up music that's suitable for her? Put it on. (Many toddlers love classical music if they have the opportunity to hear it.) Let her dance if she likes to do that. Sing together, even if it's "La la la." Tell her stories. If she's talking well, let her tell you stories if she wants to. If she likes to draw pictures, get her a pad of BIG paper and some washable crayons so she can make pictures for her family while she's right near you. (Don't give her markers or anything that could make a real mess - just a little mess.)
The idea is that while you are holding Little Brother, changing him, feeding him, rocking him, you are strengthening the bonds of communication with your daughter. You're not neglecting anybody. And Little Brother isn't going to mind a bit that you're talking big-kid matters with Big Sister while Mama is cleaning him up.
When you venture out with both your children, plan to take more than twice the amount of time than you're used to (or than you think you ought to take). Don't try to be Supermom. Don't do too many errands at a time - maybe two at the most - or just one is fine. Here, too, you talk Big Sister through it. "Little Brother can't do anything for himself yet, so let me put him in the cart first with his car seat. Then I'll put you in, and you can help me find the crackers. "Do you remember what a 'C' looks like? Let's find C-for-crackers. ...That's the box we like to take home! What color is it?"
We're all cheering you! Before long you'll wonder what you were so worried about, and you'll be encouraging other mamas.