She needs to get out of the house. Get in a routine. Her local library should have a toddler story time, ours is usually on Tuesdays at 10 or something.
Chick fil A has theme nights once a week, your local one probably has an event calendar. She may have a splash park or something in her area too. Really, getting up in the morning, taking a shower and trying to stay on a schedule will help her stay sane.
For example, breakfast, then a puzzle, then a little craft or play dough, then lunch, then nap, then do a preschool alphabet activity, put on a little movie while she tends to the baby...
Maybe she can get in with a mom's group and switch play dates, or even enroll the older child in a 2 day a week mother's day out program if you can afford it.
Also, maybe give her a few nights a week so she can go out and run errands, just to get out of the house.
If you can afford it, join a gym or the ymca, they even have flexible payment options based on your income. They have child watch for 2 hours every day, so she can get out, take a class, excersize, swim, get in a routine and have the kids being watched at the same time. Doing this, will help her keep on a routine and it will give her the much needed energy and break she needs.
My husband helps me with laundry, cooking, cleaning, taking care of the kids when he gets home and it's so nice and makes family time more enjoyable. We also rotate who does the whole bedtime routine every day. He also gives me a lot of encouragement on days I feel like throwing in the towel and revamping my resume.
Encourage her to get a fun hobby, a craft or reading or something.. and to stay off the computer ;)
Let her know you appreciate her and go out on a date once a week. For us, that usually means put the kids to bed early, make snacks and netflix a favorite tv show or movie and watch it together. About once a month, we will get a sitter or switch kids with another couple and go out on a kid free date.