I am in Bridgeport and Mathew Hamming just married. Mathew has a kid from an old girlfriend. I added the kid to Mathew's household before the marriage so that she would go to his wedding and meet his new wife and step kids. After they were married my child sim did not have the family icon next to her new step family in the relationship panel. The kids from the new wife they do show Mathew Hamming as their step-father but have no familial relationship to their new step sister. What is going on? Can I fix this?

Hi Tommee1 - your post kind of got burried here. Not sure if this is what you are looking for but have you tried this?
Adding Or Removing A Parent, Child Or Sibling Within A Household
You will need Master Controller and Master Controller Cheats
In Master Controller, select: Sim > Advanced > Family

It is my understanding that using those methods would result in my sim losing the relationship with her mom and having her step-mother added as her mother. I am wondering if I can somehow add the step family relations? Mathew Hammings new wife has kids and they all show Mathew Hamming as their stepfather but they only know his daughter as an acquaintance instead of step-sister.

I believe this is a shortcoming (to put it mildly) of the weak way in which stepfamily relations are handled in the game. There is no way to explicitly designate a sim as a stepparent, stepsibling, or stepchild. The designation, when it does show up and it doesn't always do so where it should, does so indirectly as a result of sims with kids marrying each other.

Ok I guess I just have to live with it. Thanks

Tommee1:
I tried it using cheat.

1st, I used Debug Enabler to do Make Sim to girlfriend for Mathew Hammings.

2nd, using Master Controller, I did "instant baby" for Mathew Hammings and his gf.

3rd, using Master Controller, I did trigger age transition to baby and made her toddler.

4th, I used Master Controller to make them ex girlfriend/boyfriend.

5th, I again used Debug Enabler to do Make Sim, made new female and male.

6th, I used Master Controller and made new female and male to become wife & husband.

7th, I again used Master Controller, I did "instant baby" and made twin baby sons for this new wife and husband.

8th, using Master Controller, I did trigger age transition to two twin baby sons and made them toddler.

9th, again I used Master Controller, I did trigger age transition to two twin baby sons and made them childern.

10th, Using Debug Enabler, I did instant kill to make husband sim die so that I can make his wife single/ex-wife status.

11th, then I saved again for last time before trying to make Mathew Hammings and this female sim.

12th, finally I tried using Master Controller to make Mathew Hammings and female sim wife & husband.

13th or lastly, I checked the relationship panel and viola!!!!!! All of them, Mathew Hammings, his blood related daughter, wife, and wife's two twin sons, they all had proper relationship icons. I didn't have any problems that you were having.

@bn4mtyg8d - That's great. But it doesn't change the fact that the step-sibling relationships do not always show up for all players when they should and there really isn't any way to adjust that like we can with other family relationships using MC. This has been reported upon for years.

update: when Mathew Hamming and new wife had a baby, the game updated all family relations in relationship to the new baby and it then added the relationship to the step-siblings. Just a flaw in the game, but you never know when the game will decide to correct things I guess.

Yeah! Reason Number 3,679 that our sim households need to be having more babies, having more (and more) kids just fixes everything. And welcome to Fertile Falls.