To be a credit to something - a family, a company, a university - is to reflect well on it, to make it look better, to bring honour or regard.
If she is a credit to her family, whoever is saying that believes that she brings credit to her family - not in terms of money, but that she shows that her family are in some way good.
Cambridge put it thus: to be so good or successful that the people or things that made you successful should be mentioned.
I'm not sure it's always used purely for things that contributed to that success, but it may well be that the meaning was originally limited in that way.
So, if her family say it, it means they are proud of her because her success makes them look good. If someone else says it, it means that her family ought to be proud of her.
It can be used in any tense, just modifying the verb to be appropriately. So if someone says it in the present tense, without any qualification, they either mean it as true right now, or as a general statement (in this case, because it is about a person rather than an activity, that general statement would usually include 'right now').