I know this is long, but if nothing else, please read the last paragraph.
I'm a firm believer that things happen in God's time. I have some good friends that started trying to get pregnant, the same time that my husband and I did. I was about 4 months along when she miscarried. . . for the first time.
Over the next four years, they went through various tests, IUI's, IVF's, one tubal pregnancy, and countless miscarriages. They were exhausted - physically, emotionally, and financially.
They had given up. They were going to take at least a year off of the IVF treatments to rest her body and pay the mounting bills for prior treatments. In the mean time, her husband started his own business from the ground-up.
A few months later, she went in for her annual checkup, and was asked the standard question, "when was your last period?" It had been 4 or 5 weeks earlier, and she thought nothing of it, because a 5-6 week cycle was not uncommon. Her OB/GYN snuck in a pregnancy test, and low-and-behold, she was indeed pregnant.
Six years later, they have two healthy, happy girls (both conceived naturally), and they'd be happy if a third comes along. They know now that they were trying to get pregnant on their own terms, in their own time, not God's time. (And I think it helped that they took the pressure off of themselves, kind of like the people that get pregnant when they go on a romantic cruise or adopt a baby).
This may or may not be God's time for you. If it is, you shouldn't feel guilty about taking attention away from your brother-in-law and his girlfriend. (Two babies are even better than one!) But if it's not time, don't be discouraged, and please don't beat yourself up every month when your period comes or the test reads negative.
Enjoy this time with your daughter, because when a second baby comes, it changes things - BIGTIME. And if a second one never does actually come, I promise that your daughter won't be scarred for life. Seeing you and your husband stressed and unhappy will do far more damage than growing up without a sibling. VERY IMPORTANT: Take time regularly to reconnect with your husband. Remember that you married him -for him- and that babies are just a bonus.