Obsessing about a thought

How do I stop obsessing about a thought? I have a tendency to worry and when I recently saw a family member I was concerned since they had put on a lot of weight. I know there are things they can do to live a healthier life and I want to call and offer my help since we both have Hashimoto’s which can make it difficult to lose weight.
C – they gained a lot of weight
T – they are going to have a major health issue if they don’t lose weight
F – Worried for this person and our family
A – Unmotived and feeling sick over this
R – Will cause my own health issues