It always felt like the most important thing. Wanda knew that safe wasn't always possible when you were in the life they were in, safe was sometimes impossible because they tried to make everyone else safe. But happy. That was the important thing, finding happiness in the life that they did lead, in who they knew and what they had.
Wanda didn't know a lot about the multiverse, she was still learning through the people coming here from another version of their worlds. But getting to know Steve had her feeling like he was still Steve. And she could understand why Billy felt like she was just the same as the woman he knew as his mother. "I have lost so much family, my parents, my brother. If you think of me like that, then I wholly embrace it." There was little question that she felt a connection, one that just cemented the understanding that he was something very important to her.
If he was happy for her to just slot in there, if he already considered just the same as in his universe she was happy to embrace him as her son and not put any of the alternate or other world additions on it. "We'll have a lot to explore, why put boundaries on anything."