There is a fair amount of uncertainty on this site about whether you are likely to be allowed entry. The regulations are vague:
(b) Readmission after departure to contiguous territory or adjacent island. An alien admitted to the United States under this part may be readmitted to the United States after a departure to foreign contiguous territory or adjacent island for the balance of his or her original Visa Waiver Pilot Program admission period if he or she is otherwise admissible and meets all the conditions of this part with the exception of arrival on a signatory carrier.
(Source: 8 CFR §217.3)
Note the word "may."
Explanations of this rule are also somwhat vague:
If you are admitted to the United States under the VWP, you may take a short trip to Canada, Mexico, or a nearby island and generally be readmitted to the United States under the VWP for the remainder of the original 90 days granted upon your initial arrival in the United States. Therefore, the length of time of your total stay, including the short trip, must be 90 days or less. See the CBP website. Citizens of VWP countries who reside in Mexico, Canada, or a nearby island are generally exempted from the requirement to show onward travel to another country* when entering the United States.
Note again the word "may." The effect of this word is to give fairly wide leeway to the border officer who is admitting you to the US. This means that we can't make accurate predictions about what will happen.
The most favorable fact for you is that you are not returning after a short trip. In fact, you are not returning during your "original ... admission period." So, if the officer admits you, it would have to be for a new admission period.
But on the other hand, if the officer judges that you are trying to abuse the VWP, even if that judgment is incorrect, you will be denied entry. If you're inclined to worry, or if you can't bear the thought of being denied entry, you should apply for a B-2 visa instead.
As Crazydre points out in a comment, howewver, the B-2 visa option is not without its risks:
Just Keep one Thing in mind: they'll put you under even closer scrutiny if you go to the embassy for a B2 visa (when applying for a visa, you're always put into closer scrutiny than if using the VWP), and furthermore, if you're denied a B2 visa your chances of getting a visa in the future will be severely hurt, and you'll never be able to use the VWP again, whereas if refused entry under the VWP (which I doubt will happen if you make sure to bring all documentation to Show if requested, such as your niece's birth certificate) you simply have to go for a visa in the future.