Adopting a 15 & 16 year old - a little long

My husband and I are adopting two brothers, aged 15 & 16, out of foster care. They are precious guys! Just a little background information: I never wanted to adopt. I was happy with our daughter, an only child. However, I felt the Lord tugging at me and looked at an adoption website on April 12, 2011. I saw these two boys and felt God tell me they were going to be our sons. To make a VERY LONG story short, CPS took their sweet time, but my husband and I continued to fight and never gave up. CPS finally told the boys about us on October 16, 2012. We met them October 29, 2012. The oldest stayed the following weekend with us. The youngest decided he didn't want to be adopted. The oldest told us that weekend that he wanted to be adopted. He was to stay with us the following weekend. That Thursday, they were removed from their foster home for several reasons. The youngest decided to stay the weekend with us, too, mainly because he was removed from the foster home. They were moved into a respite foster home just five miles from our home. They previously lived about 90 miles away. We got the boys every weekend and holiday including all of Thanksgiving. We picked them up every Tuesday, Wednesday and Thursday for various church activities. The boys finally were placed in our home for adoption on December 14, 2012. We were thrilled! Both boys want to be adopted. I realize they haven't been in our home long. They haven't even known of us for long. We had a year and a half to grow to love them, and I have to admit, I love them very deeply...unconditionally!! They are AMAZING young men, and we are very blessed to call them our sons. Right now, I am struggling with their lack of love for us, especially the oldest. The youngest has done a complete 180. He said he would never call us mom and dad...he started calling us mom and dad after being in our home for two days. He said he would never change his name...he wants to change his name to our last name. He never seemed happy or smiled before. He is ALWAYS happy now and smiles all the time. We also get lots and lots of hugs. The oldest is very happy, too. I just haven't seen as big a change in him. He is going to hyphenate his last name, and that's fine with us. We understand that. While in the respite home, the boys created a FB page and contacted their biological parents, a big no-no per CPS. Instead of making them delete their account, we have allowed them to keep it as long as we have access to it. After all, they are teenagers and could very well go behind our backs and create another account that we wouldn't know about. We'd rather know than not know. They talk to their parents every now and then. We know they love their biological parents very much. Their mom is very supportive of the adoption. It's hard for me because I feel like I'm in competition with the biological family. It's especially hard right now because my husband is away on business for a month, and he hasn't been here to be my rock, and our daughter is away at college. So, it's been me and our two sons alone together for the first time. I guess my question is this...how long does it take for teenagers to grow to love you? How long does it take for them to truly consider you their parents? Our adoption consummation is in April. Yippee! But I know that doesn't mean that we will be their parents in their hearts. I'm not naive, and I'm not expecting to be loved overnight. I'm just wondering how long it will take. Will they be adults before they truly realize how much we love and adore them? I tell them I love them. The youngest has begun to tell me he loves me, too. The oldest doesn't. Should I stop telling them I love them? Do you think it makes them uncomfortable if they don't feel the same way? I don't know what to do, and I don't want to push them away by loving them too much. Does that make sense? Any help, advice and/or encouragement is greatly appreciated.