Mimikyu are unfortunately very shy by nature, anonymous; each one simply expresses it a different way. In the case of yours, that means obvious, extreme shyness—hence her tendency to hide when faced with her own teammates.
One way you can help the rest of your team bond with her is by initiating and overseeing group activities. The more your mimikyu sees you interact with your other pokémon, the more likely she’ll relax when in the presence of her. Encourage her (by speaking to her) to interact with your primarina or tsareena alongside you. Note also that this doesn’t necessarily have to be in battle, either. Virtually any activity you can think of can be used to help bridge the gap between your mimikyu and the others. Play with your team, have them do housework together, or simply go on a leisurely walk—whatever is easiest for you.
Additionally, ensure that everything your primarina and tsareena do around her (even when you’re not present) are non-threatening, even on an unintentional level. Have them keep their distance at first as they communicate with her, and make sure they invite her to join them, rather than let them approach her and try to coax her into participating with them. Teach them non-threatening methods of communicating, including open gestures and softer voices, and be sure they understand the difference between asking your mimikyu to join them and pushing their friendship onto her.
Finally, it may help to have mimikyu interact with only one of them at a time. Two may be intimidating to her, especially given the fact that both tsareena and primarina are generally much larger than the average mimikyu. Dim the lights as well in order to make your mimikyu feel comfortable.
Keep in mind that no matter what you do, it will take time for your mimikyu to open up to the rest of your team, even if she has already been a member of your family for a while now. Just be gentle as you approach her and do all that you can to ensure she doesn’t feel threatened, and let her open up in her own time.
Best of luck, anonymous!