Recently, I've traveled to Spain & Germany with Multi-Entry Schengen Visa and my visa issued by Spanish Embassy. Received Entry & Exit stamps in Spain on my passport, then flow to Germany to visit my brother in law, but there is no stamp on my passport to prove my trip to Germany. I'm preparing my paper work to apply for UK Visitor Visa soon, Can I use hotel bills, Boarding Pass, and purchase bills that I still have from my German trip as evidence to support application?
Decision-makers at the British consulate are smart enough to know about how the Schengen zone works and understand that there may be evidential shortfalls for inter-zone travel.
The way to declare this type of journey into the Schengen zone is to gang it up, here's an example...
...and in the free-form section, you can make an entry like...
...that's all they need in order to tie out your declarations to your passport stamps. If you think your family connections in Germany are material you can list those in the same section.
Can I use hotel bills, Boarding Pass, and purchase bills that I still have from my German trip as evidence to support application?
Nothing stops you, but that sort of evidence isn't generally fungible and is seen as a lot weaker evidence than passport stamps and bank statements showing withdrawals commensurate with your trip expenses.