With my oldest, I went back to work when he was 3 months old, and it was way too soon. I had a bad recovery and a rough time with breastfeeding, and looking back I probably should have just quit my job! I ended up having to quit anyway before he was a year old.
With my 2nd baby, I had been job searching until I was showing too much and gave up looking. Then I started looking into direct selling, and finally settled on Mary Kay. It's been a perfect fit.
If your sister is home during the day, you could have a set time for her to watch the baby in another room so you can work, even if it's only 45 minutes at a time. Or your baby might be happy in a sling or carrier while you work, so you could have your hands free. I usually take the time when my daughter is nursing to make phone calls, because I know she'll be quiet!!!
How old is your other child? My kids are exactly 3 years apart, and I ended up putting my son in preschool 2 days a week so I have more time to get things done, around the house & for work. If that's not an option you can always fine small ways for her to "help' with tasks and keep her busy while you get stuff done. I would have my son run to get me a clean diaper or a burp cloth, he was so proud of himself!