Many female geese are not happy polygamists. Moreover, geese less than 2-3 years old are still mentally juveniles even though they have physically reached sexual maturity. Remember that wild geese do not reach sexual maturity until at least their second year. The instinct of a 1-year-old geese is to seek a leader. Depending on whether they got the rebel gene or not, and whether they feel parental rejection, this will be either the parental figure who raised them, or another dominant leader figure (in natural circumstances a male from the gaggle with good standing). Because domestic geese are usually sexually mature at 1 year, the two instincts confuse and the chosen leader is also treated as a mate, even if they are a human or the biological father (in natural circumstances geese normally do not seek sexual interactions with their siblings or parents). Once they reach emotional maturity the confusion usually disperses.
One approach is simply to wait until they are all emotionally mature and see if things resolve of their own accord. Another solution is to bring Poppy a mate of her own. The best way to do this is for the established couple to raise a male gosling (introducing an adult male will be trouble). Once the male gosling matures he will propose to Poppy, and she will then become accepted as a daughter-in-law. This is a far safer route than introducing more adult geese.
Another approach is to remove the gander for a few months, such that loneliness will cause the two females to bond and become best buddies. When the gander is reintroduced he may get the cold shoulder for a while, but after a few weeks everything should thaw out. Geese are less opposed to their buddies sharing their hubby than they are to other females who they dislike.
As a general rule with geese, you either have 1 male or more than 2, preferably at least 3. This reduces the polarization of the politics. Geese have social media too ;-)
Another general rule is that adding extra geese usually complicates life, especially if space and access to food and water are restricted.
