If your mother is offering to pay for a maternity nurse I think you'd be foolish to pass up the chance of some proper qualified help.
I had a maternity nurse for the first three weeks, but to be honest I didn't use her at all in the day, just at night, so wasted my money, but that was because dd was my first, I had no other commitments so had the time and energy to look after her myself.
To clarify what a maternity nurse does, they work over a 24 hour shift, and you agree with them the hours that they will work, and when they will take their breaks. Mine slept with the baby in her room at night, bought her into me for bf-ing, and then took her afterwards and settled her back to sleep. This meant that I actually got a decent night's sleep every night, rather than spending hours pacing with a windy baby.
She also would have worked shifts in the day had I required her to.
It is an adjustment having someone in your house, but maternity nurses are professionals, and are sensitive to giving you and your family privacy. I couldn't fault mine, she was a pleasure to have around.
If you would like your maternity nurse to help with your older child rather than the baby some of the time, that is something to discuss with them at interview. Mine had just come from a placement where she had care of three small children, obviously not at the same time, but she basically picked up where the mother most needed her.
Call the agency and speak to the manager, who will forward you the CVs of those maternity nurses who would be happy to fulfil your brief.