Labs are very high energy dogs you can't possibly leave them all day and expect them to behave in the home - expect inappropriate chewing and other boredom/stress related behaviour. So yes you would need to have them walked in the day, in addition to you walking them first thing and then after work.
How old are the dogs?
Please be very sure of how this is going to pan out before you commit yourself to these two dogs. Please don't do what your sister has done, take them on and then want to rehome them when they become an inconvenience. It's unfair to push them from pillar to post, they need to be settled in a long term home, and two labs are going to be an awful lot of work.
I don't understand how people get high energy dogs when their home situation is such that the dog will be alone most of the day. In my opinion it's not fair on the dog. I'm lucky as I can take mine to work with me everyday.
Splitting the two puppies up would be distressing for them and as a pair left alone all day they will probably wreak havoc. I'd recommend she finds someone who can take them both and who can give them a home environment that will keep them mentally and physically stimulated with human contact for most the day.