Q.F.
S.,
Ditto to all the responses provided to you already! My 2 year old had a momentary relapse as well when her little sister arrived. As already stated, you just have to continue to put in time with your oldest and reassure her with hugs and kisses and encouragement that you understand she had an accident. Mom and Daddy still love her. Physically get down to her level and kindly affirm her feelings and instruct her like you did before when you first started potty training her. Being proactive vs reactive goes a long way in changing behavior. Remember parents have the unique combination of influence/pressure and unconditional love but how you apply that pressure & love is all up to you!
Note to you: FRUSTRATION is a part of parenting so take a step back when you feel your level rising. BREATHE and lovingly and patiently try again. One thing that my husband and I do is mantras or make up songs. ('Saw this make up a song technique on Elmo's World!) We teach our oldest a short sentence or song to say when she needs to correct a behavior and constantly repeat with animation--humor/wittiness goes a long way to defuse; we also reward with stickers ($1 at the dollar store), even locate library books to read to reinforce ideas. Once upon a potty is a good book and pushes literacy.
e.g. Song or Mantra we use using the melody of gingle bells - I peepeepee peepeepee peepeepee in the potty. Mommy peepeepee peepeepee peepeepee in the potty. Daddy peepeepee peepeepee peepeepee in the potty. We all peepeepee peepeepee peepeepee in the potty.
Even if they don't do it; keep saying it so that your oldest will get the message to sit her bottom on the potty. And singing crazy songs will certainly make you laugh too.
Now lets sing - 5, 6, 7, 8 . . . I peepeepee peepeepee peepeepee in the potty. Mommy peepeepee peepeepee peepeepee in the potty. Daddy peepeepee peepeepee peepeepee in the potty. We all peepeepee peepeepee peepeepee in the potty.