Legal Question in Insurance Law in Maryland

Generally, yes, someone can designate whatever beneficiary(ies) they choose. Note however that one cannot entirely disown a spouse -- if the spouse is getting too small portion of assets, they may have a right to seek certain statutory rights to 1/3 of the total estate.

Also note that while insurance benefits will not count as probate assets, they do count for purposes of calculating death taxes. A spouse does not pay taxes for property they get from the other spouse while a sibling may, depending on the amounts and means of transfer.