I have always found that giving my dog with seniourity all of the attention instead of the new puppy helps.. When I brought Bentley home, I already had Kallie and Stewie and neither one of them liked him.. I just put Bentley's bellyband on him and let him run around the house while I showered Kallie and Stewie with lots of extra love and kisses.. It was funny, they would look at him like "ha! mama don't like you either" It took a couple weeks and now a yr later, they are inseperable... Bentley and Stewie have to cuddle all the time and then to see them argue is just hilarious...Kallie is very easy going but if she had to choose, she would have me all to herself....I just brought Maddie home and Kallie is a little resentful right now (Bentley and Stewie love her) but I again just give Kallie all of the extra attention...Just give Marley time and she will be fine....Dogs are very forgiving....
