Hi Nadz,
I sincerely mean it when I say that it's very admirable for you to want to take on all the household responsibilities and let your MIL rest. And i even understand your point of view when you say that you'd be upset if your own brother's wife did not help around in the house.
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BUT*........to simply say how you would treat in-laws.........and to **ACTUALLY* live with in-laws are two different scenarios and can't be compared. We all like to say that we would do things a certain way..............and then when we are actually placed in the situation, we find that it is the opposite of what we had imagined. Saying is different from doing. Experience provides a bit more reality and hard-core insight into a situation that we might have imagined to be much easier and more simple than it actually is.
My grandmother lives with my chachi, who is her bahu. My grandmother is pretty old and has several health problems but she still insists on doing some of the cooking and washing the dishes. If one INSISTS.........it's harder to take the task away from them and give them a break. Also, many elderly people (including in-laws) CHOOSE to engage in some work around the house because it keeps them busy. Not only that, staying active is important in old age as there is a greater risk for health problems.
I know MILS who are okay with their DILS working. They KNOW that when the DIL is away at work.............it is challenging for her to come back from work in the evening and start on chores. So the MILS help their DILS with the cooking while theyr'e at work. And they split up the chores and compromise. That's the more important issue here. Sharing and compromise.
Keep in mind that when we don't live with someone..............we don't know about the tiniest of dynamics in the relationships. Now, I'm not trying to scare you........but I'm just giving an example. Let's say that you try your best as a working wife to take care of your in-laws and ALL the household chores. But maybe you forgot to clean a spot on the floor. Or perhaps you forgot to wash a dish when you were making up all the bedrooms. What if your MIL was to pick on you for these little things.......not considering that you're trying your best. What if your MIL told your husband's sister......that "Nadz is lazy, she doesn't clean the floor properly and is careless about the dishes. All she cares about is going to work." Would it be fair for your sister-in-law to say that you deserve to be kicked in the teeth? What about your side of the story?
What I'm trying to say is that 1) You won't know how you'll act with in-laws until you're actually living with them....... 2) There are always 2 sides to a story...........3) Too many dynacmics going on in a relationshp that can swerve the balance sometimes so there needs to be flexibility............4) Roles within a family are not always black and white, some blurring does occur. Desi society often makes household chores the sole responsibility of the wife, but a woman might need help from her husband or in-laws if she's working, or tired, or out of town. Even the Prophet SAWS did his own chores in the house (mending his clothes, for example).
Since you plan on working after marriage, your in-laws will have to be flexible. And hopefully they're realize this without much hassle. It will be tough for you to handle work and take on every little household responsibility. If your MIL wants to help you out.......let her. You'll appreciate the help. You'll find out more when you start living with them. Right now, it's too early to say how things will be. But, it's good that you want to enter this living arrangement with a strong and positive attitude. And you sound so much more happy and confident. InshaAllah everything will work out great, girl! :)