Kristin - I was 32 when I married my hubby, who had a 5 year old girl. His ex had walked out and moved in with another man, taking daughter with her, about a year before I met him. I got pregnant and we married after the baby was born (we both wanted the baby very much) Our son is now 19, SD 25 married with two kids.
Unfortunately, relationships with stepchildren can be very volatile. My SD asked the day my son was born "when that baby is going to die". She knew what she was saying. I almost postphoned my wedding because of her. But I bit the bullet and married my husband because I loved him very much, and wanted to be a good step-mom to his little girl.
Fast forward 19 years later. My step-daughter has hated me for the last 5 years, ever since she met and married her husband. (you can read my story if you're up for an epistle)
The best advice I can give you, in retrospect (i.e. what I wish I had done differently)
1) It is not fair for your in-laws to expect you to love this child right away. Love grows with time. What you have to do is be FAIR with her. And don't expect her to love you right away. BUT, INSIST (and this is the important part) that she RESPECTS you as your husband's wife.
2) Don't try to parent her. Leave this, as much as you can, to her father. Don't take abuse in any way, or any behaviour that you would not tolerate from your own child. Let her know that you are her elder, and adult, and as such, deserve respect in your home.
3) Don't focus too much on what your SD calls her BM boyfriends. That is up to the BM. As for what she calls her bio dad, that is UP to her bio dad, not you. If he is prepared to tolerate it, then don't let it bother you. Personally speaking, I would simply refer to him as "Dad" or "Daddy" when you reference him. If he chooses to respond using his name (and dad accepts that), don't worry about it. If he grows up hearing him called dad or daddy by you and your daughter, he will likely follow in this direction.
4) Establish a set of rules for your house. Remember that you can't undo behaviour in two days what is learned in the other five; however - kids are VERY smart. They soon learn that there can be two very different sets of rules and expected behaviour in different houses. If he talks back saying "We don't do that at mommy's house", then just politely point out that in yours and daddy's house, we have different rules (or do things differently) and keep those expectations up.
5) Try to spend some quality time with your step-son doing something he likes, even if it is only 15 minutes a day. Most children of that age love to be read to. Have storytime with him when he's with you. He'll learn, and come to enjoy that quiet time.
6) Give yourself a break and don't try to love him. But be kind. You can't expect the maternal feelings you have for your daughter to be there right away for your step-son, as much as you WANT them to. If you learn to respect each other, love will likely come. Even if it doesn't, as long as respect is learned, you won't have as many problems as he grows up.
7) As far as your family is concerned, take the same attitude with them. Tell them that you don't expect them to love this little boy right away, but you do expect them to treat him well. And especially, don't allow them to show preference for one child over the other when BOTH are together, in the form of gifts etc. If they are giving a gift to the baby, they should have some small thing for the step-son. Little kids feel this one big time. One thing I will credit my family for is that they ALWAYS treated my step-daughter as part of the family. No, they didn't spend the same $$ on them - $$ isn't the point. The thought is what matters. This little boy is undoubtedly confused by the parade of people in his life. Let him know that you're not going anywhere, and he can come to you if he has a problem.
Have patience. It truly IS the biggest virtue. And go easy on yourself. No one is the perfect step-mom. All that is important is that we try our best.