(1) The expression “self-serving” is, to a large extent, a protean one. The expression itself tells us little or nothing. What is needed is a reason, however brief, for that designation. For example, a letter written by a third party to an applicant for international protection may be “self-serving” because it bears the hallmarks of being written to order, in circumstances where the applicant’s case is that the letter was a spontaneous warning.
(2) Whilst a statement from a family member is capable of lending weight to a claim, the issue will be whether, looked at in the round, it does so in the particular case in question. Such a statement may, for instance, be incapable of saving a claim which, in all other respects, lacks credibility.
It would be better to avoid the phrase completely in my view. And presenting evidence from family members is a classic “damned if you do, damned if you don’t” with both Home Office and many judges. If it is presented, it is given little to no weight despite the time and resources that have to be invested to acquire it, usually on a legal aid fixed fee. If it is missing, it damages the case. The latter is obviously worse than the former but some realism and self awareness from decision makers would be welcome.