This is really not an easy question to give guidance on.  If your Father believes that your Mother is acting in such a way as to encourage immoral behaviour between yo and your boyfriend or allowing someone to stay in the house and thereby taking away from the necessities available to you and your siblings (adding another mouth to feed) then your Father can try and attack your Mother morally and try and sway the judge with the issue.  It is not unheard of.  If the Judge will agree with his position really depends on the Judge and the facts.  I would indeed consult with an attorney in your area on the matter.  While they are negotiating the custody agreement slip this in as a topic for discussion with the attorney.  You and your siblings are indeed old enough to be heard by the Judge as to whom you wish to live with.  But it is the Judge who will make the final determination as to what is in your "best inerests."  Good luck.