My mother at the suggestion of her realtor signed a sale contract of a property that is in my name only. The realtor was aware that my mother was not on the deed. Is the contract valid?

Well... first... you'r mother can't deliver a property she doesn't own... being her name is NOT on the deed.

But there could be complications related to non-performance ( hopefully someone with more experience can either back me up or correct me) it if someone wants to buy it and is unable.

The realtor if they knew this like you said... did something unethical... and I would report them to the licensing agency for your area.

Your mother should call them and ask them to cancel this for those reasons... if they are smart they will realize the ramnifications and do it.

I think the real question here is whether you (the property owner) are OK with the contract or not. If your mother signed on your behalf because you weren't available the contract is (in my opinion) invalid. As noted it by smoothy it puts the real estate agency at risk, as well as your mother for having improperly represented herself as the owner, and also gives the buyer the opportunity to back out if they discover the misrepresentation. However, if your intent is to sell the property, and you're happy with the contract as written and plan to abide by its terms, does it really matter?