When two people enter into a long term relationship, it is expected they will tend to be less available to their family. Seeing them on weekends or holidays is a natural expectation, and those in the relationship might call them during the week when they have a few minutes. In a normal relationship, each of them will take the time to converse with family members on a regular basis. Isolation from family occurs when one or both members of the relationship make a firm decision to cut ties with their families.
Being isolated from family has happened many times in history, but it often had to do with moving far away and having few good lines of communication. Those who moved for a spouse were often restricted to writing letters, and they seldom had an opportunity to go home for a visit. It was an accepted part of life, but modern communication has made it easier.
The partner who is looking for a way to escape from their own family will often find a way to isolate their partner from their family as well. Cutting all family ties is their way of ensuring their spouse will cling to them, and it is a controlling behavior. The person who falls for it is generally looking for acceptance by their spouse, and they often fail to realize what they have given up until it is too late.
Those who choose to let themselves become isolated from their family are giving up a large part of their life. While it is expected they will spend the majority of their adult life away from family, there are still good times and relationship progression they should be able to enjoy with those who grew up with them. For someone to give all that up for a partner, their choice could be one that leaves them lonely in their old age.