legacy status

Your son will get a bump in his chances of getting in if your husband/his father graduated as an undergraduate. The fact that your daughter attends there doesn't give him an extra bump. If she was in fact considered a legacy student and got a bump, then the same sorts of considerations will be granted to your son.

Some schools have very narrow definitions of legacy such as Harvard which only considers "legacy" as offspring of direct graduates of Harvard College, not their other Graduate Schools like HBS, HLS, HSA, etc. Other universities have different tiers of legacy and grandparents are part of the legacy family as in the case of USC.

To get an exact idea of what boosts a legacy gets from a particular school you have to research that school and see if they publish in their online newspaper or do press releases or have stats on their website with regards to legacy enrollment. Also, keep in mind that some schools are dropping legacy preferential treatment like #9 Johns Hopkins University that no longers give legacy applicants any additional consideration. After Michael Bloomberg gifted the school $2 Billion dollars primarily for financial aid of deserving students, their coffers are full and have no reason to fill seats of legacy students that may not have the same level of academic accomplishments as some first-gen/low-income students who otherwise may not have been admitted in the past because of a lack of funding. About 3/4s of Top Tier colleges still acknowledge legacy applicants but as they get richer and richer, it's harder for them to justify keeping those policies in place when there are record amounts of applications. At MIT, they received 66% more applications than last year, have nearly $20 billion in endowment, and no legacy policy. CalTech and the UC system in CA have no legacy policy either and that doesn't seem to affect the publics' demand for applying there.