Dreams often reflect our inner thoughts, feelings, and relationships. In your dream, several elements stand out that could provide insight into your current emotional state and family dynamics.

1. Home and Family Dynamics: The vacation home symbolizes comfort and relaxation, while the transition to a family member's home suggests a shift in dynamics or unresolved issues within your family. This could indicate feelings of nostalgia, longing for family connection, or anxiety about family relationships.
2. Sister-in-Law with Ballooned Belly: The imagery of your sister-in-law with a painful-looking belly that can rotate may represent underlying tensions or conflicts that are being ignored. Her nonchalance could indicate a sense of resignation or acceptance of a difficult situation. This might relate to how you perceive her role in the family or how you feel about the family issues you mentioned.
3. Bonding with Your Niece: The connection with your niece signifies a desire for closeness and support within your family. It suggests that despite the tensions with adults, there is still a strong bond with the younger generation, perhaps highlighting the importance of nurturing those relationships.
4. Grocery Store Scene: The grocery store commonly represents the gathering of resources or preparing for life's needs. The concern about the suspicious behavior of your sister-in-law's mother may reflect your instincts about trust and safety within your family. This might indicate a fear of betrayal or a desire to protect your loved ones.
5. Urgency and Departure: The urgency to leave in a vehicle could symbolize a desire to escape potential conflict or danger. It may also reflect a need to take control of a situation or protect your family from perceived threats.

In summary, your dream seems to reflect a mix of familial concerns, unresolved tensions, and a longing for connection. It could be beneficial to reflect on your feelings towards these family members and consider whether there's a need for communication or resolution of past issues. Engaging in open discussions with family members might help alleviate some of the underlying tensions you feel.