My sister has been trying to have a child for the last year. she has had a stable partner for this period of time but it seems like she can not get pregnant. what should they do to have a baby?

See OB/GYN for eval: It is recommended to see a professional if a couple was not able to conceive a child after a year of unprotected sex (6 months if a woman is >35 years old). To increase chances of conception the couple should have sex 2-3 times a week, optimize medical conditions, normalize body weight (bmi 19-26), reduce stress, avoid smoking and alcohol, limit caffeine consumption and start early (age < 30).

Multiple things: First try to predict when ovulation is occurring. There are phone apps and ovulation prediction kits available. Then try to have sex when ovulation is occurring. Have sex regularly especially during times of ovulation keeping in mind that sperm concentration will decrease each time. There are treatment options for people who can't conceive.