Here are some suggestions on how to address this situation:
Communication: Encourage open communication between the siblings. Talk to both of them separately to understand their perspectives and feelings. Help them express their emotions and concerns without judgment.
Respect boundaries: Teach your younger sibling about respecting personal space and boundaries. It's essential to respect when someone doesn't want physical contact, even if they are family.
Negotiation: Help your children find a compromise. Perhaps they can agree on some activities they enjoy doing together without physical contact, or maybe the older sibling can set aside some time to spend with the younger one, which may alleviate the desire to sit on her lap constantly.
Positive reinforcement: Instead of using tickling as a response, which could lead to more conflicts, praise and reward positive behaviors and respectful interactions between them.
Redirecting behavior: If the younger sibling continues to sit on the older one despite her discomfort, gently redirect their attention to other activities or spaces where they can play together without causing distress.
Supervision: Depending on their ages, you may need to supervise their interactions to ensure they treat each other with kindness and respect.
Family discussion: If the issue persists, involve the whole family in a discussion about respecting each other's boundaries and finding ways to foster a harmonious relationship between siblings.
Remember, sibling dynamics can be complex, and conflicts are a natural part of growing up. Patience, understanding, and guidance from parents or guardians can play a significant role in helping children learn to navigate these challenges and build healthy relationships with each other.