Unfortunately, I don't think Sieg is the father lol. With the way they talk, it just seems like they're reallyyyyyyyy close friends/sworn allies.

: Friendzoned forever...
But there seems to be a bit of a hint that Hilde might not survive the next game and her children will replace her.
"Refusing to fall, Hilde immediately assembled her troops alongside Siegfried, anxious to put an end to the demonic evil and finally bring peace upon Wolfkrone. Deep within her soul, Hilde knew that she may not live to witness her subjects' safe return home. However, her two children might."
If she did die, one of her babies will inherit her weapons, I'm sure. Her style is too beautiful to be lost <3