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.
Well, it could lead to a scolding. Mom might be really angry and ground them for a while. For example, if they were doing something naughty like sneaking cookies before dinner.
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 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.
There could be a story where the sister and brother are practicing a dance routine for a family talent show. The mom catches them rehearsing and they stop, a bit embarrassed, but then show her what they've been working on. It turns out to be really fun and creative.
She could show immediate pride and give them hugs. For example, if they are reading books together to help each other with schoolwork, she might say 'I'm so proud of you two for working together like this.'
Being disrespectful is another reason. If you talk back rudely to your sister or don't listen to your mom, you're likely to get punished. Say you roll your eyes at your sister when she asks you to do something simple like pass the salt, that's disrespectful. Your mom might ground you and your sister might stop sharing her things with you.
One common reason is disobeying house rules. For example, not cleaning your room or staying up too late.
One possible reason is that the sister might have a more assertive personality. For example, she could be born with a natural leadership quality that makes her take charge. Just like some people are more outgoing and confident from a young age.
Maybe she was short on girl models for her sewing projects and thought her son could be a good stand - in.
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.