I'll start by saying I'm fully prepared to be told I'm unreasonable and constructive criticism is fine. This is also a really long post!
DH and I have been married a few months. Before we were married there were no real issues with dss 7 or ds 13. Dss is 40/60 with us.
Mine and DHs parenting seemed to match up in most ways and blending our family before marriage was surprisingly challenge free. Its now not and I'm struggling and DH is struggling and so are our DCs. I also have another dd 13 so there's 3 in total. My dcs df is a bit shit and contact is inconsistent but overall my dc are well rounded happy dc and whilst they're sometimes annoying and cheeky they know where my line is.
We've now come to the realisation that our parenting is actually different. Things that annoy me don't annoy DH and vice versa. Obviously our dc have been brought up with what the parents annoyances are and moulded that way.
There's three main issues.
One: It really irritates me to my soul that DH ask DSS if somethings OK before he does it. Eg - yesterday we had seperate days out with our DC. I went on a bike ride and DH went on a walk. Halfway through my bike ride DS chain snapped and DD wasn't very well. DH was already home as he had gone out earlier than us. I cycled home and got my car and came back for DC. DH rang me and I told him what was going on and that I couldn't get DDs bike in my car (new bike for Xmas). He said he'd come and get us in the van and then asked dss if that was alright. Then, as I hadn't had any lunch and was freezing cold I asked DH to take DD back with him so I could go through Costa drive through. He said yes but then double checked with his ds if that was OK. It really irritated me and I'm not sure why and its something I've been noticing lately. This is different parenting styles I guess but I don't understand why you would double check with your seven year old if its OK to come and rescue your wife and step children. What if he had said no 
Two: Dss manipulates DH. He didn't want to go for a walk Saturday morning so pretended to his DM that he didn't want to come over as he feels sad and misses having his dad to himself. He most likely does feel sad and confused ect but this was done to get out of something he didn't want to do. DH tries to take him out for bike rides, tries to play minecraft, tries to get him to read for 30 mins every night, wants him to help cook - the list goes on and DH offers are refused. However, when he's in trouble or doesn't want to do something he cries about missing dad. DH feels guilty about this and doesn't really know what to do about it. It doesn't help that DSS DM and DH do not get on so they can't work together as a team. His DM has told him things like - I'm scared you will love them more then me and made it very clear she doesn't like me. Lots of conflicting loyalty issues for dss.
Three: Ds and Dss bickering and play fighting. I'll start by saying my DS is a wind up merchant and takes things to far. He has a group of 'bantery' friends and can be a nause. He's also kind, thoughtful and really funny when it doesn't go to far. Dss adores DS 95% of the time and DS has given up his bedroom to share with DS 3 nights a week (whilst we convert loft for him). The issue is when one of them (usually my ds but not always) goes to far. I think its normal sibling behaviour and both have consequences for their actions. DH now doesn't think this is enough as my ds is 13 and his is 7. He feels like DS should say - go away you're 7 and not get down to his level. I agree and don't agree. On one hand yes DS is 13 and should do what we ask which is tell DSS to go away or come and tell us. I also feel however that DSS shouldn't start things in the first place and it's unfair on my DS when he does. Me and DH are now locked in a battle between the DS's on whats acceptable and what's not. We've put in a three strike rule for pocket money but the line between playing and it going to far is grey and neither of us are consistent with it. DSS line of whats acceptable and whats not is different then DS. Dss ran in my room and shouted at me to shut up the other week. He told his dad it was a joke and DH agreed it was a joke but told him to not joke to me like that again as its not very respectful. If he 'jokes'like that to my DS I can see why my DS responds back without thinking. My DS jokes are not as funny to DH as they are to me so again it could just be how we've moulded our dc different.
Both of us think the other is to soft on our DS's. My DS is a proper boy, he's tough in lots of ways (although hates being told off and is finding our new set up really hard). DSS is not like that. He has so little confidence, he won't be left alone to have a shower/put in petrol, he won't try anything new and is naughty is a subtle way with his behaviour to get out of what he doesn't want to do whereas my DS was a naughty in a loud boisterous way when that age. I do think DH is a bit wet with his consistency. I understand to a degree, he is desperate to be a part of his sons life and is terrified that DSS won't want to see him anymore if he's to strict so his discipline is at his discretion. Eg when its just him and his son he doesn't use the 3 strikes - whereas mine who live here full time are on it all the time. DH thinks that's right as he doesn't see his ds every night of the week.
I feel trapped in a battle of petty things and not feeling happy when dss is here. I feel on eggshells and that its DH and dss against me and my dc. He even said in our argument last night that he's only thinking about him and his kid. He backtracked completely and said it came out wrong but he still said it. I don't know where we've gone so wrong. Dss doesn't like me anymore - last night he told me he hoped I'd go off with my friends all the time and not be in our home - DH doesn't notice these types of comments...
I really don't know where to go from here. I've gone from being deliriously happy with my new family to walking on eggshells and feeling dread the day dss is due to come. What can I do to make this better? Me and DH do talk and things do get better for a few days but it always happens again and we both get defensive about our dc and entrenched in our positions.