Well, it could be that mom is feeling lonely or her own children are growing up and leaving home. So she wants her sister to have a baby to bring back that sense of family togetherness. It could also be that she sees her sister as a great potential mother and thinks a baby would complete her sister's life. However, these are still mom's own projections and not valid reasons to force the sister into pregnancy.
This is a very complex situation. Generally, it is not ethical. Pregnancy should be a personal choice based on one's own will, not someone else's desire. In most cases, forcing or wishing someone to get pregnant without their full consent goes against basic human rights and respect for individual autonomy.
One possible reason could be a need for attention. Sometimes children act out in unusual ways to get their parents to focus on them. If the girl feels neglected, she might think that this extreme behavior will make her mom pay more attention.
The tiny little sister might be caught because she was being naughty outside. For instance, she might have been told not to go near the pond in the garden, but she went there anyway. Also, she could be caught if she was tattling on her siblings when she was the one who started the trouble. Mom would soon catch on to her false accusations and catch her in the act.
Misbehavior towards others is also a reason. For example, if the brother and sister are fighting with each other or being mean to their neighbors or friends, the mom may feel the need to discipline them firmly. Also, lying can be a cause. If they are caught in a lie and the mom wants to teach them a lesson about honesty, she might spank them. However, it should be noted that these are traditional reasons and in modern parenting, there are better ways to deal with these issues.
It might be a bigger pond. A larger pond can attract more wildlife like frogs and dragonflies. She can also put more water plants in it and create a more diverse aquatic ecosystem in the garden.
It could be a misunderstanding. Maybe he was just using it for some art project at school where he needed different fabrics and the panties were just one of them.
Maybe she was short on girl models for her sewing projects and thought her son could be a good stand - in.
One reason could be to help him with a bad habit. For example, if the son bites his nails, the mom might hypnotize him to stop this behavior. Hypnosis can plant a subconscious suggestion to break the habit.
Another reason might be that they are not following the rules. For example, if there is a rule that they can't watch TV until they finish their chores, and mom catches them slacking off and watching TV instead, she would have to step in and remind them of the rules.
One possible reason could be accidental pregnancies. Maybe the daughter was not careful enough, and at the same time, the mom was also in a situation where she unexpectedly became pregnant. Another reason could be differences in life choices. For instance, the daughter might be starting her family early while the mom decided to have another child later in life.