Does the sole provider of a single income household become resentful?

So it turns out that my sister-in-law has been offered a full scholarship to finish her PhD. Currently she’s been working part-time and going to school part-time. At this rate it will take her 8 or 9 years to complete all of her required courses. Being able to dedicate a couple of years to completing her degree will certainly help her reach her goal more quickly. But at what cost?

My brother is trying to determine whether or not they could live solely off of his salary for the next couple of years. So here’s my question for all of the single income families out there. Do you think a resentment will grow between my brother and his wife if she is able to quit her job and return to school full time? After all, at the end of three years she will be able to pursue her dream of being a professor. But my brother will still be stuck in the same job.

My brother and his wife have two children under the age of two. Going to school full time may allow her to spend more time with her children. Thus preventing them from paying for costly day care. But how will my brother feel after a long day of work? Will he look at my sister-in-law with resentment for being able to pursue her dreams, and enjoy time with their children, while he spends his day in a thankless job we does not like?

My husband and I do not have any children of our own, but my husband believes most of his co-workers, who are the sole providers of single-income families resent their roles. Obviously you want your spouse to be happy, but misery does love company.