!!!! Help With Electrical and Tiling Job Customer Is Saying I'm Charging Too Much !!!!

Before I start this my first post I'm not to sure if this is the right place for this post if it isn't can someone let me know and I apologise in advance

I was asked to install 5 x Electrical points 4 internal 1 external the internal points needed chasing for the backbox and cable the external point needed chasing for the cable Surface mounted Point

Also I was asked to Tile kitchen splash back the area to be tiled is 7 Meters in length in sections and around 27 square metres I have some images of before and after I have  Indicated the internal points I installed with white arrows in a couple of the images

I only provided labour

I have done work for the customer Previously and everything has been fine

We did not Discuss a rate of pay but they know I'm fair with my rates or at least I Thought I was

I know your Thinking BIG SCHOOL BOY ERROR  you always discuss your rate of pay there are good reasons why I  didn't think I needed to

One of the reasons why is the customers partner is a Qualified electrician Who does the job for a living so he knows the value of the job and the other reason is the customer is my sister

I sent an invoice for works completed as requested by my sister's partner
I got a Reply  !! lol is that the discounted price?

The invoice was for £600 in total £200 for 5 x Electrical points £400 for kitchen splash back

I do not Believe I am charging too much.I believe I m charging too little because it's my sister
I also feel that my work is to a high standard and value for money and that this devalues my work and workmanship

If anyone believes I am charging to much.I would really appreciate it if you let me know.And if you believe i'm charging too little again , let me know