First of all, the background was an my attempt of doing a brighthouse indoors.

Now context, let's say it's a sequel to the drawing of Misty and alphabittle.
After misty went back in time to the first thanks giving to get Turkeys of the menu... wait wrong movie.
After Misty went back in time and found young alphie, she brought him back to the future with her so she'd still have her alphie while haven has hers.
A few weeks later Misty noticed that Sunny was feeling a little down, so she asked what was wrong.

"Is that seeing you reunite with your father, reminds me of when mine was alive..."

Misty understood that sunny was depressed because she missed her father, and since Sunny's birthday was close she decided to make her a little surprise.
She went back in time to when Argyle was alive befriended and told him why she was there etc.

Unlike her case with alphabittle where she was gone, thus bringing him to the future wouldn't affect the outcome of this timeline.
Argyle had Sunny, though she was still a newborn and without a mother, so Misty brought Newborn Sunny and Argyle back to the future.

Time skip
"I missed you so much dad"
Said sunny burying her face on his chest with tears in her eyes.
"It's okay sunny bunny, I'm here now"
He said hugging and cumforting her.