This is Gearhead and his live-in not-sibling, Silver Link, below is how they met.
 
It had been a rainy summer day. The weather crew was making up for several dry days with one big storm. Too big, as it left everyone in home waiting it out. At least for those who actually KNEW it was going to happen. One mare had been unlucky in missing the memo and was now trying to seek shelter from the elements.  
“Hello, is anyone in here?” She asked, knocking a hoof against the door to a workshop. “Please, anyone!”  
“Nopony’s here, go away!” Came the voice of a grumpy-sounding stallion.  
“That statement is antitehtical!”  
On hearing a rather large word, the door opened and the mare found herself being glared at by a stallion with mechanical front hooves and wings. “Only because you seem smart.” He stepped aside and let her in.  
“What is this place?” She asked as she looked around.  
“My workshop, I build many, many things for everypony.”  
The mare looked at a small music box, starting it up with her magic. “Ooo.”  
“What’s your name, filly? And what’s with the Cutie Mark?”  
“Silver Link, and my talent is finding how things connect.”  
The stallion raised a brow.  
“I didn’t pick it! Otherwise I would’ve had a normal one, like music or animal care.”  
He chuckled. “Gearhead.”  
“Your name?”  
“Duh.”  
“Fitting for your Cutie Mark.”  
“Double duh. Want coco?”  
“Yes please.”
 
After the coco was brewed and poured, the two began to talk. As it turns out they hit it off rather well, what with their similar interest in seeing how things work together or how one thing can be similar to another. On the talk of family the mood because more sad, as Gearhead couldn’t really remember his family, and Silver Link had moved out some time ago and missed her own clan.  
“So if you live by yourself, how’re you paying for your home?”  
“Trying to find friends for those really shy ponies. An odd field of work, but it pays well sometimes.”  
He just gave her a blank stare.  
“Like I said, I didn’t choose it!”  
“Do you room with anyone?”  
“Well…no.” She looked down into her drink. “Been so busy finding friends for everypony else that I don’t really have time to think of that kinda stuff.”  
“Rent is paid by the end of the month, and you get whatever you need for yourself.”  
“…Wait, WHAT?”  
“Congradualtions, you’re my roomie.”  
“But…I’m a mare! And you’re a stallion!”  
“We’re too busy with our respective fields for others to think otherwise.”  
“…”  
“See you bright and early.”  
And Silver Link was left baffled as to how in the WORLD this happened. To this day, she still is. Perhaps her magic had kicked in at some point and she didn’t know it? Regardless, she still loved her BBLBFF, her big brother-like best friend forever.