Is my mom’s will valid?

It depends in large part on how you signed it. A person who is of sound mind may direct that a will be signed for her, if she cannot physically sign it, but generally, the person signing must also indicate on the will (typically under the signature line) that she or he signed for the testator (person making the will)--that is, you can't simply write your mother's signature, but must clearly indicate that you are signing for her. A failure to do so may make the signature invalid, because it purports to be something it's not: the actual signature of your mother (if you signed her name without indicating that you did so).
Also, even if the signature is technically valid, that does not stop your sister from attacking the will on the basis that your mother was not mentally competent when it was made; other common attacks she could make in this context would be that used duress against your disabled mother to make her sign, or used "undue influence" (a high degree of control you had over her life and access the world), or simply that you did not actually have her instructions or directions to sign for her.
To win on those challenges, your sister would have to have evidence to prove that one or more of those things happened, which she may not have; and you could present your own evidence to support the will (e.g. the testimony of the witnesses, who can testify that your mother asked you to sign). But be aware that even if the signature is, again, technically valid, that's not the end of the story: a challenge my be possible. Given that you signed, that the will was made just before the end, and presumably that the will benefits you more than your sister, there are facts that could suggest impropriety--at the least, your sister could, if she chose, state a good enough case to get into court and get a hearing on the subject.