I assume this would count as her "annual paid" vacation?
Even though it was spur of the moment?
Regardless, you need to clarify that with her... what is going on in her mind about it? You have to ask her.
Does she expect to be paid? Because it was last minute.
If it were me... I would assume this is part of her annual vacation, paid. But that has to be agreed on.
BUT the bottom line is... what is stipulated in your "contract" or "agreement" of services with her? Then you go according to that.
Being she is such a GOOD RELIABLE Nanny... then I would work something out with her. Fairly. Even if it just half-pay.
BUT, she did get you a "replacement." So perhaps in lieu of that, you don't "have to" pay for her spontaneous 'vacation.' BUT, you BOTH have to talk about it and agree on it. Perhaps she does not even "expect" to be paid, anyway?
In a similar situation, I used to babysit... and when "I" took my annual vacation, the parents STILL paid for the month. As usual. But I had a contract that stipulated that, and which the Parents signed off on. But I gave them a big lead-time and head's up as to when I was taking my "vacation" time. AND, when the Parents suddenly decided to take a "vacation", they STILL paid for the month, as usual, in full. BECAUSE, I had to essentially "reserve" and guarantee their child's spot in my home-care babysitting. This is typical of babysitters.
As far as if you go into labor "early"...well, your Hubby I'm assuming will take off from work from that point, right? Most Husbands do.
Then, he will have to mind and care for your 2.5 year old son. For me, BEFORE my due date... I made out a "list" for my Hubby...so that he KNEW what the "routines" of my daughter was, and what to do, even bedtimes, what she likes to eat, and her pre-bed/nap routines etc. I CLEARLY delineated all of that, so that my daughter would not be stressed, having me not around and being in the hospital having my baby (my son). THIS helped, my Husband. PLAN PLAN PLAN, ahead... for any scenario. AND it helped my daughter. I even explained to my daughter that Mommy would be in the hospital and Daddy would be with her at home... and he will know what to do.
THEN, when your other family arrives in town... your Hubby will have "help." THEY should be helping... not being a "guest" and with you taking care of everyone. AND, you "can" still have your Nanny come in, maybe half-time... since your son is used to her, and she can help, if need be. BUT, only if this is agreed to. Just another way in which to look at this.
For me, when my "family" was in my home, after me & baby was home... it was harder for me actually, because all my family did was play with baby and see baby. Me, I was STILL doing it all! My Hubby took time off from work, so he was still the MAIN support/help with my daughter, while I recovered from my c-section.
Anywho, well sorry for rambling and going off subject. Just some thoughts I had.
Make sure you plan ahead for any scenario. Sometimes, having lots of family in the home, can be "more" distracting than "help."
All the best,
Susan