This can be a difficult thing for a lot of people. Most of the people I know (non-writers mostly) don’t know when you should capitalise words like mum, dad, grandma, grandpa, uncle, aunty, and any other variations of these words (eg: ma, granny, pa, papa, unck, grandfather, etc).
There is an easy answer to this, and an easy way to remember which one to use. The rule is this: if you are saying “my mum”, “my dad”, “my uncle”, and so on, you don’t need to capitalise. You are telling us the relationship you have with these people. On the other hand, if you are calling them mum, dad and uncle in place of their name, then you would use a capital letter. For example, “Hey, Dad! Dinner is ready!”You are calling him Dad instead of his given name. It is a title, just like his actual name, so, just like his actual name, you give it a capital. Or “Let’s go to Grandma’s house for lunch!” She is your grandma, but you call her Grandma.
I hope this has been helpful to you. Let me know in the comments.