Being in a long term relationship means sharing everything, and that includes family. Couples planning important celebrations often involve them, and some of these events might even be for family members. Loved ones can generally be counted on to help with preparations, but those who are jealous of outside affection are often busy pushing family away. They might be subtle in their actions, but the effect is to isolate their partner whenever a large event occurs. It can be a successful ploy until their partner realizes what is happening and decides to do something about it.
Some families are very close, and it can create havoc within a relationship. Jealousy is often how it manifests, but anger can be another emotion that gets out of control. The partner who is always inviting their family over to help with special events might suddenly find they are hesitant to be there, and it is a painful realization. Their partner might be insulting them whenever possible, or they could be sabotaging the relationship in other ways.
Isolation from friends and family is difficult for someone who loves social events, and they could find it painful to be caught in the middle. Their relatives might suddenly begin suggesting changing the hosting of events, or they could have a tepid response whenever it is suggested a celebration be held. It is often a slow process, but a jealous partner can often accomplish isolation within only a few months if they work hard at it.
Turning the situation around will mean confronting the person doing the damage, and they couple will have to figure out why it is an issue in the first place. Simply assuring a jealous person they have no reason for concern does not always work, so consulting a professional might be helpful if they are to keep their relationships intact.